pbkdf2: make public PBKDF2 Ident values

[ChanTsune]

For RustCrypto members

Thank you for developing a awesome crate!

I have a question.

Why do the other crates maintained by this repository have published Ident values, but not in pbkdf2?

Looking at the code, it seems that there is an intention to publish it, but it is not available because there is no use declaration.

If you just forgot to publish it, I would appreciate it if you could merge this PR and make it available.

[tarcieri]

Before exporting these I think we might consider better names that reflect they're Idents.

Perhaps either adding *_IDENT to their names or making them inherent constants like Pbkdf2::SHA256_IDENT.

[ChanTsune]

Thanks for your advice!
I added _IDENT as a suffix to each.

[tarcieri]

Personally I think I'd prefer the inherent constant approach, since it cuts down on the number of exports/imports

[ChanTsune]

OK!, so how about making it an intrinsic constant of Algorithm following the other crates as follows?

https://github.com/ChanTsune/password-hashes/blob/ca2ff2abdfa347ce7d6a3e4bd764f988ba9a282c/pbkdf2/src/simple.rs#L82-L94

[tarcieri]

That looks good, yes

[tarcieri]

Looks good, thanks!