scrypt: Modular Crypt Format (MCF) support

[tarcieri]

Currently the scrypt crate only supports the PHC String Format, the default used by the password-hash crate.

Ideally it would additionally support MCF as used via the crypt(3) interface, where password hashes have a $7$ prefix. See also:

https://man.archlinux.org/man/crypt.5#scrypt