Add `syscall/js` compatibility in the "What's new" section.

Merge pull request gopherjs#1028 from nevkontakte/master

Update Go version in CI to 1.16.4 and prepare for a new release

Increment GopherJS version to 1.16.1.

Merge pull request gopherjs#1015 from gopherjs/go1.16-stdlib

Add support for Go 1.16.3 standard library

CI: fix breakage and update minor Go version

The latest version of node-gyp (v7.1.2 at this time) is not compatible
with Node v10.0.0 that circle.yml installs. Go back to a known working
version of node-gyp to get a working CI.

Update the CI script to test against Go 1.12.17¹,
the latest minor release in the Go 1.12.x series.

¹ https://golang.org/doc/devel/release.html#go1.12.minor

Updates gopherjs#894.
Fixes gopherjs#987.

GitHub-Pull-Request: gopherjs#988

compiler/natives/src/sync: update for Go 1.13 internal API changes

runtime_SemacquireMutex and runtime_Semrelease had a new parameter
added in Go 1.13. Modify the signature of our override to match.
The parameter is not used, since it's for tracing, which is not
something GopherJS supports at this time.

Add victim and victimSize struct fields, also new in Go 1.13, to
our Pool struct override.

Fixes:

	$ gopherjs build sync
	/goroot/src/sync/waitgroup.go:93:36: too many arguments
	/goroot/src/sync/rwmutex.go:133:44: too many arguments
	/goroot/src/sync/rwmutex.go:103:49: too many arguments
	/goroot/src/sync/rwmutex.go:85:44: too many arguments
	/goroot/src/sync/rwmutex.go:50:49: too many arguments
	/goroot/src/sync/pool.go:242:5: p.victim undefined (type *Pool has no field or method victim)
	/goroot/src/sync/pool.go:243:5: p.victimSize undefined (type *Pool has no field or method victimSize)
	/goroot/src/sync/pool.go:248:5: p.victim undefined (type *Pool has no field or method victim)
	/goroot/src/sync/pool.go:249:5: p.victimSize undefined (type *Pool has no field or method victimSize)
	/goroot/src/sync/pool.go:168:31: p.victimSize undefined (type *Pool has no field or method victimSize)
	/goroot/src/sync/pool.go:168:31: too many errors

compiler/natives/src/sync: update for Go 1.13 internal API changes

runtime_SemacquireMutex and runtime_Semrelease had a new parameter
added in Go 1.13. Modify the signature of our override to match.
The parameter is not used, since it's for tracing, which is not
something GopherJS supports at this time.

Add victim and victimSize struct fields, also new in Go 1.13, to
our Pool struct override.

Fixes:

	$ gopherjs build sync
	/goroot/src/sync/waitgroup.go:93:36: too many arguments
	/goroot/src/sync/rwmutex.go:133:44: too many arguments
	/goroot/src/sync/rwmutex.go:103:49: too many arguments
	/goroot/src/sync/rwmutex.go:85:44: too many arguments
	/goroot/src/sync/rwmutex.go:50:49: too many arguments
	/goroot/src/sync/pool.go:242:5: p.victim undefined (type *Pool has no field or method victim)
	/goroot/src/sync/pool.go:243:5: p.victimSize undefined (type *Pool has no field or method victimSize)
	/goroot/src/sync/pool.go:248:5: p.victim undefined (type *Pool has no field or method victim)
	/goroot/src/sync/pool.go:249:5: p.victimSize undefined (type *Pool has no field or method victimSize)
	/goroot/src/sync/pool.go:168:31: p.victimSize undefined (type *Pool has no field or method victimSize)
	/goroot/src/sync/pool.go:168:31: too many errors

ci: upgrade to Go 1.11.5 (gopherjs#903)

So that we can then see the true diff that the Go 1.12 changes in
gopherjs#900 will introduce.

compiler/prelude: Keep nil slice nil when slicing. (gopherjs#852)

According to the Go specification:

> If the sliced operand of a valid slice expression is a nil slice,
> the result is a nil slice.

This behavior was not implemented and not caught previously.
Slicing a nil slice would incorrectly make it an empty but non-nil slice.
This change fixes that.

The minified prelude was regenerated with:

	go generate ./compiler/prelude

Fixes gopherjs#851.

internal/sysutil: Fix build error on FreeBSD due to Rlimit.Cur type m…

…ismatch. (gopherjs#752)

Document why type conversion is necessary. Otherwise, it's not clear
that it's neccessary, and we we wouldn't be able to catch breakage if
it were removed.