Newly internal declarations include Hasher._seed and the integer overloads of Hasher._combine(_:), as well as _SipHash13 and _SipHash24.
Unify the interfaces of these SipHash testing structs with Hasher. Update SipHash test to cover Hasher, too.
Add @usableFromInline to all newly internal stuff. In addition to its normal use, it also enables white box testing; compile tests that need to use these declarations with -disable-access-control.
Beyond switching hashing algorithms, this also enables per-execution hash seeds, fulfilling a long-standing prophecy in Hashable’s documentation.
To reduce the possibility of random test failures, StdlibUnittest’s TestSuite overrides the random hash seed on initialization.
rdar://problem/24109692
rdar://problem/35052153
