Merge in upstream changes

[tmthrgd]

This is a continuation from https://github.com/tmthrgd/go-hex/issues/1#issuecomment-385658982.

Of note are the following changes:

• golang/go@22cfe24aca80653b0e8efdd4a1aba1df00e8e72d: encoding/hex: save allocation in DecodeString(), (I tried this and the benchmarks showed the reverse happened, but maybe I screwed up the benchmarks somewhere).
• golang/go@6db4950dc57deb899bf5550411c01ce32f16bdd0: encoding/hex: add NewEncoder, NewDecoder, (File this under nice-to-have).
• golang/go@d0ce197c5835941df6d1d06017c1c34866a13650: encoding/hex: make Decode, Decoder, DecodeString agree about partial results and errors, (This one isn't clean and it isn't easy. It interacts badly with the assembly code, see https://github.com/tmthrgd/go-hex/issues/1#issuecomment-385658982 for what it might look like).