refactor: move stuffer hex methods out of testlib

[lrstewart]

Description of changes:

We need to be able to read / write hex for JA4. In this change, I take the testlib/s2n_stuffer_hex.c methods and clean them up for non-test use:

• I deleted the methods that weren't being used and weren't needed for JA4 (the uint32 and uint64 versions). They're easy enough to add back later if we need them, but removing them reducing the testing area for now.
• I rewrote the s2n_stuffer_read_hex and s2n_stuffer_write_hex methods to be lighter weight and only do one stuffer read / write
• I made everything return S2N_RESULT

Call-outs:

This PR doesn't include the CBMC proofs we need for stuffer methods. It was already pretty large, so I'll put the proofs in a follow-up PR. I've written them, and they're passing locally.

Testing:

I also rewrote the tests. I didn't think the existing tests were clear enough about what was being tested, and there weren't enough known-value tests.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.