Add P-H-C string-format support to `scrypt`

[brycx]

scrypt can have a safe_api/alloc gated PasswordHash type (similar to that in orion::pwhash) that implements parsing to/from P-H-C password hash strings.

• https://github.com/P-H-C/phc-string-format/blob/master/phc-sf-spec.md
• No official documentation of P-H-C parameters for scrypt seem to be available, but most implementations I've observed seem to follow a layout as implemented here: https://github.com/simonepri/phc-scrypt