Add alloc feature for easier lib usage

## Description

Continuing the discussion from https://github.com/iotaledger/crypto.rs/pull/3 - crypto.rs should provide an `alloc` feature that allows opting in to more convenient usage patterns (eg. returning `Vec` 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.