Cherry picked from master at 129d3035

Also ran gofmt on everything. Thanks again @paralin. #334

Co-authored-by: Christian Stewart <christian@paral.in>

Closes #242

NetConn has to close the connection to interrupt in progress reads
and writes. However, it can block reads and writes that occur
after the deadline instead of closing the connection.

Closes #228

This way we don't pollute the directory tree.

Closes #182

Closes #179

Too many improvements and changes to list.

Will include a detailed changelog for release.

See https://github.com/golang/go/issues/31586#issuecomment-550040882

Thanks @renthraysk

benchmark                      old MB/s     new MB/s     speedup
BenchmarkXOR/2/fast-8          470.88       492.61       1.05x
BenchmarkXOR/3/fast-8          602.24       719.25       1.19x
BenchmarkXOR/4/fast-8          718.82       1186.64      1.65x
BenchmarkXOR/8/fast-8          1027.60      1718.71      1.67x
BenchmarkXOR/16/fast-8         1413.31      3430.46      2.43x
BenchmarkXOR/32/fast-8         2701.81      5585.42      2.07x
BenchmarkXOR/128/fast-8        7757.97      13432.37     1.73x
BenchmarkXOR/512/fast-8        15155.03     18797.79     1.24x
BenchmarkXOR/4096/fast-8       20689.95     20334.61     0.98x
BenchmarkXOR/16384/fast-8      21687.87     21613.94     1.00x

Now its faster than basic XOR at every byte size greater than 2 on
little endian amd64 machines.

Closes #168

Closes #164

- For JS we ensure we indicate which size initiated the close first from our POV
- For normal Go, concurrent closes block until the first one succeeds instead of returning early

I changed my mind after #103 as browsers include a wasClean event to indicate
whether the connection was closed cleanly. From my tests, if a server using
this library prior to this commit initiates the close handshake, wasClean
will be false for the browser as the connection was closed before it could
respond with a close frame. Thus, I believe it's necessary to fully implement
the close handshake.

@stephenyama You'll enjoy this.

math/rand was being used inappropiately and did not have
a init function for every file it was used in.

Closes #150

- Also allows SetReadLimit to be called concurrently which is a nice
  touch

Closes #153

Realized I can at least make the Reader/Writer/SetReadLimit methods
work as expected even if they're not perfect.

Removes the tiny filenames polluting the root directory.