This moves the bvec hash table logic out of Liveness.compact and into
a bvecSet type. Furthermore, the bvecSet type has the ability to grow
dynamically, which the current implementation doesn't. In addition to
making the code cleaner, this will make it possible to incrementally
compact liveness bitmaps.

Passes toolstash -cmp

Updates #24543.

Change-Id: I46c53e504494206061a1f790ae4a02d768a65681
Reviewed-on: https://go-review.googlesource.com/110176
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>