encoding/charmap: add DecodeByte, EncodeRune methods.

Also change exported values' types from encoding.Encoding (an interface)
to *Charmap (a newly exported struct type), since we can't add methods
to that interface type.

Also fix a few "go vet" issues.

Fixes golang/go#19584

Change-Id: I32f726a791f39c158a782ed013b1d358f0b77e57
Reviewed-on: https://go-review.googlesource.com/38873
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>