This adds a bitmap indexed by page number that marks the starts of
in-use spans. This will be used to quickly find in-use spans with no
marked objects for sweeping.

For #18155.

Change-Id: Icee56f029cde502447193e136fa54a74c74326dd
Reviewed-on: https://go-review.googlesource.com/c/138957
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>