Adds a new custom bitset template to handle packing as many bits as
possible into a single variable. Intelligently select the right class
depending on platform features and bit sizes.

For now, always use a packed 64-bit set on 64-bit, instead of using
a 32-bit set for smaller bitsets.

BUG=angleproject:1814

Change-Id: I3ffef815c15515555833f6fc9302d8a4eee5423b
Reviewed-on: https://chromium-review.googlesource.com/471827Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>