fix closenow race

Context can be cancelled by parent. Doesn't indicate the CloseRead goroutine
has exited.

See #326

Far simpler now. Sorry this took a while.

Closes #427
Closes #429
Closes #434
Closes #436
Closes #437

Not ideal but whatever, I'm going to rewrite all of this anyway.

Fix unaligned load error on 32-bit architectures

Closes #432

On some 32-bit architectures, 64-bit atomic operations panic when the
value is not aligned properly.

In this package, this causes netConn operations to panic when compiling
with GOARCH=386, since netConn does atomic operations with int64 values
in the netConn struct (namely, with readExpired and writeExpired).

This commit fixes this by moving readExpired and writeExpired to the
beginning of the struct, which makes them properly aligned.

use simd masking for amd64&arm64

See https://github.com/nhooyr/websocket/pull/326#issuecomment-1771138049

Standard library does this too. Unfortunate wish they just exposed it in the
standard library. Perhaps we can isolate the specific code we need later.

I'm just not good enough at assembly. I added tests to confirm that @wdvxdr's
implementation works correctly and matches the output of the basic masking loop.

[qrvnl@dios ~/src/websocket] 130$ go test -bench=. ./wsjson/
goos: linux
goarch: amd64
pkg: nhooyr.io/websocket/wsjson
cpu: 12th Gen Intel(R) Core(TM) i5-1235U
BenchmarkJSON/json.Encoder/8-12         14041426            72.59 ns/op  110.21 MB/s          16 B/op          1 allocs/op
BenchmarkJSON/json.Encoder/16-12        13936426            86.99 ns/op  183.92 MB/s          16 B/op          1 allocs/op
BenchmarkJSON/json.Encoder/32-12        11416401           115.3 ns/op   277.59 MB/s          16 B/op          1 allocs/op
BenchmarkJSON/json.Encoder/128-12        4600574           264.7 ns/op   483.55 MB/s          16 B/op          1 allocs/op
BenchmarkJSON/json.Encoder/256-12        2710398           433.9 ns/op   590.06 MB/s          16 B/op          1 allocs/op
BenchmarkJSON/json.Encoder/512-12        1588930           717.3 ns/op   713.82 MB/s          16 B/op          1 allocs/op
BenchmarkJSON/json.Encoder/1024-12        823138          1484 ns/op     689.80 MB/s          16 B/op          1 allocs/op
BenchmarkJSON/json.Encoder/2048-12        402823          2875 ns/op     712.32 MB/s          16 B/op          1 allocs/op
BenchmarkJSON/json.Encoder/4096-12        213926          5602 ns/op     731.14 MB/s          16 B/op          1 allocs/op
BenchmarkJSON/json.Encoder/8192-12         92864         11281 ns/op     726.19 MB/s          16 B/op          1 allocs/op
BenchmarkJSON/json.Encoder/16384-12        39318         29203 ns/op     561.04 MB/s          19 B/op          1 allocs/op
BenchmarkJSON/json.Marshal/8-12         10768671           114.5 ns/op    69.89 MB/s          48 B/op          2 allocs/op
BenchmarkJSON/json.Marshal/16-12        10140996           113.9 ns/op   140.51 MB/s          64 B/op          2 allocs/op
BenchmarkJSON/json.Marshal/32-12         9211780           121.6 ns/op   263.06 MB/s          64 B/op          2 allocs/op
BenchmarkJSON/json.Marshal/128-12        4632796           264.2 ns/op   484.53 MB/s         224 B/op          2 allocs/op
BenchmarkJSON/json.Marshal/256-12        2441511           473.5 ns/op   540.65 MB/s         432 B/op          2 allocs/op
BenchmarkJSON/json.Marshal/512-12        1298788           896.2 ns/op   571.27 MB/s         912 B/op          2 allocs/op
BenchmarkJSON/json.Marshal/1024-12        602084          1866 ns/op     548.83 MB/s        1808 B/op          2 allocs/op
BenchmarkJSON/json.Marshal/2048-12        341151          3817 ns/op     536.61 MB/s        3474 B/op          2 allocs/op
BenchmarkJSON/json.Marshal/4096-12        175594          7034 ns/op     582.32 MB/s        6548 B/op          2 allocs/op
BenchmarkJSON/json.Marshal/8192-12         83222         15023 ns/op     545.30 MB/s       13591 B/op          2 allocs/op
BenchmarkJSON/json.Marshal/16384-12        33087         39348 ns/op     416.39 MB/s       27304 B/op          2 allocs/op
PASS
ok      nhooyr.io/websocket/wsjson  32.934s

json.Encoder is 42% faster than json.Marshal thanks to the memory reuse.

goos: linux
goarch: amd64
pkg: nhooyr.io/websocket/wsjson
cpu: 12th Gen Intel(R) Core(TM) i5-1235U
BenchmarkJSON/json.Encoder-12            3517579           340.2 ns/op        24 B/op          1 allocs/op
BenchmarkJSON/json.Marshal-12            2374086           484.3 ns/op       728 B/op          2 allocs/op

Closes #409

Slower for some reason than just SIMD.

Also no dependency on cpu package is nice.

goos: windows
goarch: amd64
pkg: nhooyr.io/websocket
cpu: Intel(R) Core(TM) i5-9300H CPU @ 2.40GHz
Benchmark_mask/2/basic-8         	425339004	         2.795 ns/op	 715.66 MB/s
Benchmark_mask/2/nhooyr-8        	379937766	         3.186 ns/op	 627.78 MB/s
Benchmark_mask/2/gorilla-8       	392164167	         3.071 ns/op	 651.24 MB/s
Benchmark_mask/2/gobwas-8        	310037222	         3.880 ns/op	 515.46 MB/s
Benchmark_mask/3/basic-8         	321408024	         3.806 ns/op	 788.32 MB/s
Benchmark_mask/3/nhooyr-8        	350726338	         3.478 ns/op	 862.58 MB/s
Benchmark_mask/3/gorilla-8       	332217727	         3.634 ns/op	 825.43 MB/s
Benchmark_mask/3/gobwas-8        	247376214	         4.886 ns/op	 614.01 MB/s
Benchmark_mask/4/basic-8         	261182472	         4.582 ns/op	 872.91 MB/s
Benchmark_mask/4/nhooyr-8        	381830712	         3.262 ns/op	1226.05 MB/s
Benchmark_mask/4/gorilla-8       	272616304	         4.395 ns/op	 910.04 MB/s
Benchmark_mask/4/gobwas-8        	204574558	         5.855 ns/op	 683.19 MB/s
Benchmark_mask/8/basic-8         	191330037	         6.162 ns/op	1298.24 MB/s
Benchmark_mask/8/nhooyr-8        	369694992	         3.285 ns/op	2435.65 MB/s
Benchmark_mask/8/gorilla-8       	175388466	         6.743 ns/op	1186.48 MB/s
Benchmark_mask/8/gobwas-8        	241719933	         4.886 ns/op	1637.45 MB/s
Benchmark_mask/16/basic-8        	100000000	        10.92 ns/op	1464.83 MB/s
Benchmark_mask/16/nhooyr-8       	272565096	         4.436 ns/op	3606.98 MB/s
Benchmark_mask/16/gorilla-8      	100000000	        11.20 ns/op	1428.53 MB/s
Benchmark_mask/16/gobwas-8       	221356798	         5.405 ns/op	2960.45 MB/s
Benchmark_mask/32/basic-8        	61476984	        20.40 ns/op	1568.80 MB/s
Benchmark_mask/32/nhooyr-8       	238665572	         5.050 ns/op	6337.22 MB/s
Benchmark_mask/32/gorilla-8      	100000000	        12.09 ns/op	2647.28 MB/s
Benchmark_mask/32/gobwas-8       	186077235	         6.477 ns/op	4940.36 MB/s
Benchmark_mask/128/basic-8       	14629720	        80.90 ns/op	1582.19 MB/s
Benchmark_mask/128/nhooyr-8      	181241968	         6.565 ns/op	19497.98 MB/s
Benchmark_mask/128/gorilla-8     	68308342	        16.76 ns/op	7639.37 MB/s
Benchmark_mask/128/gobwas-8      	94582026	        12.97 ns/op	9872.11 MB/s
Benchmark_mask/512/basic-8       	 3921001	       305.6 ns/op	1675.55 MB/s
Benchmark_mask/512/nhooyr-8      	123102199	         9.721 ns/op	52669.11 MB/s
Benchmark_mask/512/gorilla-8     	32355914	        38.18 ns/op	13411.43 MB/s
Benchmark_mask/512/gobwas-8      	31528501	        37.80 ns/op	13544.37 MB/s
Benchmark_mask/4096/basic-8      	  491804	      2381 ns/op	1720.39 MB/s
Benchmark_mask/4096/nhooyr-8     	26159691	        46.98 ns/op	87187.73 MB/s
Benchmark_mask/4096/gorilla-8    	 4898440	       243.6 ns/op	16817.89 MB/s
Benchmark_mask/4096/gobwas-8     	 4336398	       277.2 ns/op	14776.40 MB/s
Benchmark_mask/16384/basic-8     	  113842	      9623 ns/op	1702.66 MB/s
Benchmark_mask/16384/nhooyr-8    	 8088847	       154.5 ns/op	106058.18 MB/s
Benchmark_mask/16384/gorilla-8   	 1282993	       933.6 ns/op	17549.90 MB/s
Benchmark_mask/16384/gobwas-8    	  997347	      1086 ns/op	15093.49 MB/s

We're about 4-5x faster then gorilla now.