In bits.h, instead of relying on C++ overloading for 32/64 bits integers
(which can be ambiguous when the input isn't the exact type typedef-ed
by uint64_t or uint32_t), use templates and std::enable_if to switch
between integers of different sizes.

This means that we can get rid of an awkward sizeof check in
bit-vector.cc, which was necessary to compile on Mac.

Change-Id: Id0eaf0f855cdbd2dc4d7bc1c481037fcd9b73953
Reviewed-on: https://chromium-review.googlesource.com/543480
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46184}