There is no cache here yet. This CL defines ActionID, Hash, and HashFile,
which the new content-based staleness code can use. Eventually we'll
put a real cache implementation here, but it's not necessary yet.

Change-Id: Ide433cb449f4dbe658694453f348c947642df79b
Reviewed-on: https://go-review.googlesource.com/67311Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>