encoding/binary: remove TODO in Write and add benchmarks
    
    Benchmarks:
    goos: linux
    goarch: amd64
    BenchmarkReadSlice1000Uint8s-8           4097088               296 ns/op        3381.06 MB/s
    BenchmarkWriteSlice1000Uint8s-8          4372588               271 ns/op        3694.96 MB/s
    
    Change-Id: I5b6ef0da5052e3381ee9c714bbff541c11ed0259
    Reviewed-on: https://go-review.googlesource.com/c/go/+/246837
    Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
    TryBot-Result: Go Bot <gobot@golang.org>
    Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>
    Reviewed-by: Ian Lance Taylor <iant@golang.org>
    Trust: Alberto Donizetti <alb.donizetti@gmail.com>