blake2s and blake2b have been provided by the stdlib hashlib since
Python 3.6, and we declare 3.7 as minimum Python version supported.

When Software Heritage persistent identifiers were introduced, they were not
yet abbreviated as SWHIDs.

Now that abbreviation is growing adoption, rename some functions and types in
swh.model.identifiers for consistency:

  - PersistentId -> SWHID

  - persistent_identifier -> swhid

  - parse_persistent_identifier -> parse_swhid

Backward compatibility with previous naming is maintained but deprecation
warnings are introduced to encourage the use of the new names.

Numerous variables in swh.model codebase have also been renamed accordingly.

Also rework and improve documentation.

This ensures all instanciated model entities have valid types for attributes.

Related to T2308.

Will be used by loaders.