Since v1.2.0, the one shot sums (Sum32, Sum64, Sum128, and their seeded
and string versions) moved the input to the heap when called from
another package, so hashing a stack buffer allocated; reported by
@ncruces in #15. The input now stays on the stack, and summing a stack
buffer is about twice as fast. The []byte sums cost up to 0.3ns more
per call.

A hasher from New32, New64, or New128 created and used in one function
now stays on the stack, and so does a buffer passed to its Write.

New64 and SeedNew64 now return 8 from Size, the length of their Sum,
rather than 16. Every hash value is unchanged, but an HMAC over New64
now reports a Size of 8, so an HKDF over New64 with a nil salt derives
a different key.