The content ID will be needed for content-based staleness
determination. It is defined as the SHA256 hash of the file
in which it appears, with occurrences of the build+content IDs
changed to zeros during the hashing operation.

Storing the content ID in the archives is a little tricky
but it means that later builds need not rehash the archives
each time they are referenced, so under the assumption
that each package is imported at least once after being
compiled, hashing at build time is a win. (Also the whole
file is more likely to be in cache at build time,
since we just wrote it.)

In my unscientific tests, the time for "go build -a std cmd"
rises from about 14.3s to 14.5s on my laptop, or under 2%.

Change-Id: Ia3d4dc657d003e8295631f73363868bd92ebf96a
Reviewed-on: https://go-review.googlesource.com/69054Reviewed-by: David Crawshaw <crawshaw@golang.org>