encoding/hex: add NewEncoder, NewDecoder

encoding/hex: add NewEncoder, NewDecoder

NewEncoder returns an io.Writer that writes all incoming bytes as
hexadecimal characters to the underlying io.Writer. NewDecoder returns an
io.Reader that does the inverse.

Fixes #21590

Change-Id: Iebe0813faf365b42598f19a9aa41768f571dc0a8
Reviewed-on: https://go-review.googlesource.com/70210
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>