Enable password-hash/std feature

[li-kai]

Enables password-hash/std feature, allowing crates like argon2 to throw error up from password-hash.

This is helpful in situations where you want to bubble up the error:

error[E0277]: the trait bound `HashError: StdError` is not satisfied
   --> auth/src/models.rs:104:47
    |
104 |     let parsed_hash = PasswordHash::new(&hash)?;
    |                                               ^ the trait `StdError` is not implemented for `HashError`
    |
    = note: required because of the requirements on the impl of `From<HashError>` for `anyhow::Error`
    = note: required by `std::convert::From::from`

[tarcieri]

Thank you! This seems like an oversight.