* rlp: improve nil pointer handling

In both encoder and decoder, the rules for encoding nil pointers were a
bit hard to understand, and didn't leave much choice. Since RLP allows
two empty values (empty list, empty string), any protocol built on RLP
must choose either of these values to represent the null value in a
certain context.

This change adds choice in the form of two new struct tags, "nilString"
and "nilList". These can be used to specify how a nil pointer value is
encoded. The "nil" tag still exists, but its implementation is now
explicit and defines exactly how nil pointers are handled in a single
place.

Another important change in this commit is how nil pointers and the
Encoder interface interact. The EncodeRLP method was previously called
even on nil values, which was supposed to give users a choice of how
their value would be handled when nil. It turns out this is a stupid
idea. If you create a network protocol containing an object defined in
another package, it's better to be able to say that the object should be
a list or string when nil in the definition of the protocol message
rather than defining the encoding of nil on the object itself.

As of this commit, the encoding rules for pointers now take precedence
over the Encoder interface rule. I think the "nil" tag will work fine
for most cases. For special kinds of objects which are a struct in Go
but strings in RLP, code using the object can specify the desired
encoding of nil using the "nilString" and "nilList" tags.

* rlp: propagate struct field type errors

If a struct contained fields of undecodable type, the encoder and
decoder would panic instead of returning an error. Fix this by
propagating type errors in makeStruct{Writer,Decoder} and add a test.

build: switch PPA from Gophers dep to manual download

travis, Dockerfile, appveyor: bump to Go 1.13

Most of these changes are related to the Go 1.13 changes to test binary
flag handling. 

* cmd/geth: make attach tests more reliable

This makes the test wait for the endpoint to come up by polling
it instead of waiting for two seconds.

* tests: fix test binary flags for Go 1.13

Calling flag.Parse during package initialization is prohibited
as of Go 1.13 and causes test failures. Call it in TestMain instead.

* crypto/ecies: remove useless -dump flag in tests

* p2p/simulations: fix test binary flags for Go 1.13

Calling flag.Parse during package initialization is prohibited
as of Go 1.13 and causes test failures. Call it in TestMain instead.

* build: remove workaround for ./... vendor matching

This workaround was necessary for Go 1.8. The Go 1.9 release changed
the expansion rules to exclude vendored packages.

* Makefile: use relative path for GOBIN

This makes the "Run ./build/bin/..." line look nicer.

* les: fix test binary flags for Go 1.13

Calling flag.Parse during package initialization is prohibited
as of Go 1.13 and causes test failures. Call it in TestMain instead.

core/state: optimize some internals during encoding

core, metrics, p2p: switch some invalid counters to gauges

common, graphql: fix hash/address decoding + UI content type

* Accounts in alloc field must already exist

Note that accounts in alloc field must already exist, as pointed out by Simeon Vanov in https://gettoshare.com/2017/10/30/how-to-use-genesis-json-alloc-property/

* Change wording per PR review comment

* README: minor fixups

Dockerfile: expose GraphQL ports

Currently light.maxpeers is 100 - after this change it's 10 for non-servers.

Fixes #19820

internal/ethapi: implement fillTransaction

eth: disallow overwrite files via admin.exportChain

* les: wait all task routines before drop the peer

* les: address comments

* les: fix issue

* Change chainId in genesis block to 8888 from 0 

Change chainId in genesis block to 8888 from 0 per Moriteru in https://ethereum.stackexchange.com/a/28082/40230.

* Replace 8888 with “<arbitrary positive integer>”

Per PR review, replace Moriteru’s arbitrary positive integer 8888 with “<arbitrary positive integer>” in chainId field.

cmd, core, eth, les: support --override.istanbul

core/vm: enable istanbul EIPs in the jump table

This change adds support for gzip encoding on HTTP responses.
Gzip encoding is used when the client sets the 'accept-encoding: gzip' header.
Original change by @brianosaurus, with fixes from @SjonHortensius.

Original change by @jpeletier 

This copies cli.printHelp but changes minwidth to 38. Custom flag
code is improved to print the default value using cli.FlagStringer like
all built-in flags do.

build: gomobile automaticall adds the ios tag, don't duplicate