[Support] Integrate SipHash.cpp into libSupport.

Start building it as part of the library, with some minor
tweaks compared to the reference implementation:
- templatize cROUNDS/dROUNDS, as well as 8B/16B result type
- replace assert with static_assert
- return the result directly, as uint64_t/uint128_t
- remove big-endian support
- use LLVM_FALLTHROUGH

The siphash function itself isn't used yet, and will be in a follow-up
commit.