Add new Pin type with secrecy/zeroize wrapper

[sbihel]

Related to #126

In the end, I don't know if it's such a good idea. The code is only handling &str so it is up to you to decide if you think this breaking change is worth it to make users aware that they should be careful with their pins.

[wiktor-k]

I think it looks mostly OK. I'm a liiiitle bit worried about name confusion with std::pin::Pin struct. At least it confused me initially before I saw the alias 😅

[sbihel]

That's fair, and I mistakenly imported std::pin::Pin a few times... Here are a few options:

1. PIN but it's not really idiomatic;
2. UserPin but users might be confused and look for an SOPin;
3. Pkcs11Pin;
4. AuthPin;
5. TokenPin;
6. LoginPin; or
7. SecretPin.

I would vote for 2 but I don't have a strong preference.

[ionut-arm]

Hey, sorry for the long delay.

I think I'd go for something like 4, 5, or 6 from your list, but I don't have any strong feelings between them. The reason I'd avoid 2 is exactly what you mentioned, that it's technically incorrect and perhaps confusing, if it does cover both SO and User PINs.

[wiktor-k]

Yep. Maybe let's go with AuthPin and that's it. (otherwise the bikeshedding can take forever :D )

[ionut-arm]

That was fast! Thanks for the patch, I think at least one of your commits is missing a sign-off, though.

[sbihel]

Sorry about that, fixed

[wiktor-k]

LGTM 👍