Summary:
These new functions and 128-bit value bit operations are
expected to be used in a forthcoming Bloom filter alternative.

No functional changes to production code, just new code only called by
unit tests, cosmetic changes to existing headers, and fix an existing
function for a yet-unused template instantiation (BitsSetToOne on
something signed and smaller than 32 bits).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7338

Test Plan:
Unit tests included. Works with and without
TEST_UINT128_COMPAT=1 to check compatibility with and without
__uint128_t. Also added that parameter to the CircleCI build
build-linux-shared_lib-alt_namespace-status_checked.

Reviewed By: jay-zhuang

Differential Revision: D23494945

Pulled By: pdillinger

fbshipit-source-id: 5c0dc419100d9df5d4d9abb153b2855d5aea39e8