pbkdf2: add new wrapper functions

[newpavlov]

The ugly bounds on the HMAC functions can be improved by resolving RustCrypto/MACs#104.

Closes #335.

[tarcieri]

FWIW I've been trying to eliminate the usages of expect in documentation going forward, although the boilerplate to do so is a bit onerous:

#![cfg_attr(feature = "std", doc = "```")]
#![cfg_attr(not(feature = "std"), doc = "```ignore")]
//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
//! let res = pbkdf2_array::<Hmac<Sha256>, 20>(b"password", b"salt", 4096)?;
//! # Ok(())
//! # }
//! ```

[newpavlov]

I think expect in this case is preferred to the error bubbling since the panic never will be triggered.

[tarcieri]

Looks like Cargo.lock needs updated but otherwise this seems really helpful!