Refactoring of the pbkdf2 crate.

[jfrimmel]

This PR applies refactoring to the pbkdf2 crate, especially for the simple module.
It is best viewed commit by commit.

[jfrimmel]

Do you want to keep the minimum Rust version of 1.22? Some dependencies (e.g. rand) could also be updated, if the MSRV is bumped

[newpavlov]

Thank you! Looks good!

But I think we can add a small optimization.

[newpavlov]

I think you can remove the allocation and be compatible with Rust 1.22 by using:

let mut p = hashed_value.split('$');
let buf = [
    p.next(), p.next(), p.next(), p.next(),
    p.next(), p.next(), p.next(), p.next(),
];
let (count, salt, hash) = match buf {
    [
        Some(""), Some("rpbkdf2"), Some("0"), Some(count),
        Some(salt), Some(hash), Some(""), None,
    ] => (count, salt, hash),
    _ => return Err(CheckError::InvalidFormat),
};

Also it looks like Err(CheckError::InvalidFormat)? is discouraged (e.g. by clippy), so we probably better to replace it with an explicit return.

[jfrimmel]

Thank you for your suggestion. I will remove the unnecessary allocation (I wasn't happy with it, too) by applying your changes 😄
Unfortunately, I don't think, that Rust 1.22 will support this code as well.

Edit: but I have an Idea how to do this in Rust 1.22. I'll update this PR soon

[fubuloubu]

This will fix #17, correct?

[jfrimmel]

This will fix #17, correct?

Unfortunately no, since this PR does not increase the MSRV. This would be required to use Rand 0.7.

[tarcieri]

@jfrimmel can you rebase? we've bumped MSRV to 1.41+

[newpavlov]

I've included your refactor into #33.