password-hash: avoid `salt.as_ref()` in `PasswordHasher::hash_password_simple`

[tarcieri]

When passing a Salt or SaltString to PasswordHasher::hash_password_simple, it's presently necessary to call salt.as_ref() as the salt parameter is &'a str.

It should be possible to switch it to &'a impl AsRef<str> to allow simply passing &salt instead. However, that's a breaking change that should be bundled with other changes in the next release.

[tarcieri]

Fixed in #615