Optimize angle::BitSetIterator. 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/471827 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>