blake2b/blake2s support (#3116)

* blake2b/blake2s support

Doesn't support keying, personalization, salting, or tree hashes so
the API is pretty simple right now.

* implement digest_size via utils.read_only_property

* un-keyed for spelling's sake

* test copying + digest_size checks

* unkeyed is too a word

* line wrap

* reword the docs

* use the evp algorithm name in the error

This will make BLAKE2 alternate digest size errors a bit less confusing

* add changelog entry and docs about supported digest_size
7 files changed