refactor: clean up other hex methods

[lrstewart]

Resolved issues:

resolves #2759

Description of changes:

• s2n_key_log_hex_encode: I removed this method. It could be easily replaced with s2n_stuffer_write_hex.
• s2n_hex_string_to_bytes: This was only used in tests, so I added a new test wrapper around s2n_stuffer_read_hex called "s2n_blob_alloc_from_hex_with_whitespace".
  • I also moved the related macros to s2n_testlib.h. And instead of adding a third "S2N_BLOB_FROM_HEX" variation that used EXPECT_OK, I added a variation that can work for any return type.
• s2n_str_hex_to_bytes: I left this one be. It's used in s2nc/s2nd, which isn't supposed to use internal code. Technically we probably need a separate implementation in s2nc/s2nd.

I also cleaned up the existing s2n_stuffer_alloc_ro_from_hex_string so it better matched the blob version I added.

Call-outs:

This change touches a lot of files, but most of the changes are just renames / return type changes. If the PR is hard to review, I can make some of those changes in a follow-up PR.

Testing:

Existing tests pass. I moved the interesting tests of s2n_blob_alloc_from_hex_with_whitespace (mostly meaning the ones with whitespace) into a different test file.

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

[maddeleine]

Yay cleanups!

[jmayclin]

Nit: Move this comment to the header file so that my IDE picks it up whenever I mouse over the method?