Support "PHC String Format" for argon2(id) hashing

[DavidBuchanan314]

The reference implementation of argon2 supports an output format that encodes the hashing parameters, along with the hash itself. This is useful, for example, if you're storing password hashes in a database and you want to upgrade the parameters (or even the algorithm itself) over time.

The format is documented here: https://github.com/P-H-C/phc-string-format/blob/master/phc-sf-spec.md

The format is supported by other argon2 libraries/implementations, including https://github.com/hynek/argon2-cffi

Similar serialisation formats exist for most hashing schemes, and passlib supports many of them (including argon2).

[DavidBuchanan314]

Here's my first-pass implementation of PHC string parsing https://gist.github.com/DavidBuchanan314/09776258324bca75a6057b24e7e0a6ac

Edit: I found this standalone JavaScript implementation of the PHC format, may be useful for reference: https://github.com/simonepri/phc-format