Don't finalize in Hasher::write, but in finish instead

[ogxd]

Context

Currently, each Hasher::write implies a finalization. This is a problem because hashing strings for instance needs 2 writes (see write_str implementation), thus 2 finalization steps, which is the most expensive part of the hashing process.

Todo

Only finalize in finish

[ogxd]

This has involved a stability breaking change, so SMHasher tests must be reevaluated before merging

[ogxd]

Ok I am not sure why but stability is unchanged. Since the algorithm stability is unaffected and the public API is unchanged, I am versioning this as a patch.