(*Writer, error) if they take a compression level, and *Writer otherwise.
Rename gzip's Compressor and Decompressor to Writer and Reader, similar to
flate and zlib.

Clarify commentary when writing gzip metadata that is not representable
as Latin-1, and fix io.EOF comment bug.

Also refactor gzip_test to be more straightforward.

Fixes #2839.

R=rsc, r, rsc, bradfitz
CC=golang-dev
https://golang.org/cl/5639057