Tighten up string type representations to prevent illegal values

[Tudyx]

Fix #181

Mainly inspired by the der and the asn1_rs crates.

[djc]

This looks okay so far. One thought is that I think maybe the TryFrom impls should be for String instead of &str, so as to make it obvious an owned type is required (giving callers the opportunity to move a value rather than effectively having to clone it).

[Tudyx]

Maybe we could provide both? I think the TryFrom<&str> is quite nice for users because it allows moving from (old API)

PrintableString("BR".into());

to

PrintableString("BR".try_into()?)

If we only provide TryFrom<String>, users would be obligated to do

PrintableString(String::from("BR").try_into()?)

[Tudyx]

giving callers the opportunity to move a value rather than effectively having to clone it)

Having both would also solve this one.

so as to make it obvious an owned type is required

I see your point but I think it's okay because users are used to the standard String type which has a similar API ("foo".into() or String::from("foo"))

[cpu]

Here's a quick review pass. So far it's looking good. Thanks for picking this up

[Tudyx]

I've updated the PR with the missing methods, and integrate it with subject alternative names.
Some notes:

• All the types can be created with an UTF-8 encoded string ( &str/ String ). For instance, BmpString::try_from("❤πü2?")? . A check is performed to see if all input's characters are in the valid charset.
• Instead of providing TryFrom<&[u8]> I prefer to be explicit and use from_<encoding> like String::from_utf8

The PR is still a draft because I would like to add more tests but I haven't found a good way yet.

[Tudyx]

I rebased on top of #213. I've also cleanup the git history a bit

[cpu]

Looks good to me. Thank you

[cpu]

@Tudyx If you rebase I think this is ready for merge. Thank you!

[Tudyx]

That's should be it

[cpu]

Thanks again. I appreciate you wrangling this tech debt to unblock the feature work you were originally interested in doing. You went the extra mile 🏆