Allows BitVector to resize, updating its own data and data length to
match the new length. We can fast-path resizes which fit into the same
data length (since high bits are already zero), and replace the pattern
where a BitVector is cloned using CopyFrom.

Change-Id: If79ca782c516e93b2a27c5e335e263554d522e88
Reviewed-on: https://chromium-review.googlesource.com/539522Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46101}