archive/zip: add File.OpenRaw, Writer.CreateRaw, Writer.Copy

These new methods provide support for cases where performance is a
primary concern. For example, copying files from an existing zip to a
new zip without incurring the decompression and compression overhead.
Using an optimized, external compression method and writing the output
to a zip archive. And compressing file contents in parallel and then
sequentially writing the compressed bytes to a zip archive.

TestWriterCopy is copied verbatim from https://github.com/rsc/zipmerge

Fixes #34974

Change-Id: Iade5bc245ba34cdbb86364bf59f79f38bb9e2eb6
Reviewed-on: https://go-review.googlesource.com/c/go/+/312310


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Carlos Amedee <carlos@golang.org>