IdCred enhancements

[chrysn]

Some small fixes I found when updating Ariel OS to use the latest lakers:

• Some documentation on how IdCred is used would have helped, so I added an example that doubles as a doctest.
• Implementing that I found a typo (KCSS instead of KCCS); the fix is API compatible by deprecating the old version.
• The from_encoded_value constructor accepted some values that look like KIDs but are really longer. It may be a good idea to accept non-1-byte- KIDs, but this PR is designed for "all no-brainers so let's just merge it". I'm happy to provide longer KID support in there, but I don't know whether there's a limit on KID lengths in other places (won't hurt for this function to support more, though).
  [edit] Note that the way it is used internally (eg. decode_plaintext_3), the length is pre-checked to be correct due to decoder.any_as_encoded(), but that also means we're doing double work – still, a public function should do proper error handling.

[geonnave]

Thanks, just went through (including latest commit for larger KIDs) and is looking good. Happy to merge once CI is green.

[chrysn]

That latest commit was pushed by accident and is now in #326.

CI gets upset because apparently the doc comments get translated to C (cbindgen?), and there the compiler is not super happy about comments-in-comments (no way!); I'll look briefly into whether cbindgen can be made to play ball, but chances are I'll just remove those comments.

[chrysn]

The bindgen issue seems to be previously unnoticed; I'm adding a workaround.

[chrysn]

I think this is ready now; fixing things on cbindgen would have been excessive, so the comments in the doctest are in a less-than-ideal position as a workaround.

[geonnave]

Looks good!