aes-kw: `no_std` support + other cleanups

[tarcieri]

• Makes the Kek::{wrap, unwrap} functions take an output buffer rather than allocating a Vec<u8>, allowing them to be used in heapless no_std contexts.
• Adds an alloc feature along with Kek::{wrap_vec, unwrap_vec} functions which provided the previous API.
• Adds convenience KekAes128/KekAes192/KekAes256 type aliases.
• Makes the IV and IV_LEN constants public.
• Removes the use of hex for test vectors replacing it with hex-literal instead.
• Improves README.md and rustdoc documentation.

[tarcieri]

I think this is ready to release then?

@dignifiedquire anything else? Should I cut a release or do you want to?

[cryptographix]

Functionally, looks good. Just upd Deno PR to use latest commit, and is now passing 100% of WPT (WebCrypto/wrapKey_unwrapKey) and JOSE (AxxxKW) tests!

Great work, guys. Thanks a lot for your effort and patience ;)