Fix cuid check

[stormshield-kg]

Fixes the following test:

assert!(!cuid2::is_cuid2("aaa_1aaa"));

Also fixes some clippy warnings and a panic in debug when length=0.

[mplanchard]

Thanks for the PR, and sorry for the slow response! I was on vacation, and it's been a busy month.

Am I correct that the main change to the is_cuid() function is to ensure the CUID only contains valid base 36 characters? I think that's a good change. I see that we're using is_ascii_lowercase() now instead of checking against the STARTING_CHARACTERS. While these are equivalent, I would prefer we continue to check against STARTING_CHARACTERS, since that const is what's used to generate the CUID. Using it in the check means that if it changes at some point in the future, we won't need to change the check as well.

Regarding the change to not panic when length = 0, I'm of the opinion that a 0-length CUID is a user error and panicking is appropriate, especially given that our is_cuid() check explicitly says that anything of length < 2 is not a valid CUID. We could make the error more explicit, though. Would you mind throwing a check for that in the CuidConstructor::with_length() method and panicking with a reasonble error message if the length is < 2?

[stormshield-kg]

Fixed.