Add alloc feature for easier lib usage

[l1h3r]

Description

Continuing the discussion from #3 - crypto.rs should provide an alloc feature that allows opting in to more convenient usage patterns (eg. returning Vec<T> from a function) and to provide support for primitives requiring allocation (eg. RSA)

Motivation

This improves usability of crypto.rs and is required to support certain algorithms (eg. RSA)

Requirements

1. Add an alloc cargo feature to enable condition compilation with #[cfg(feature = "alloc")]
2. Add *_vec functions where they make sense (eg. AES_256_GCM::encrypt_vec)

Are you planning to do it yourself in a pull request?

Maybe.