Some additions / modifications to password validation

[Vlix]

I went through the validate-password code on my own laptop at some point and checked the comments, tried to make it a bit more DRY and tried to separate the policy validation from the password validation.

I'll probably do a last pass over the entire module for correctness and reader-friendliness just before we'll merge everything in.

[cdepillabout]

cc @HirotoShioi in case he wants to do a review of this as well

[cdepillabout]

I think this looks really good!

I left a couple comments, but I don't think any of these should necessarily block it being merged in!

[Vlix]

I think this should be most of the code in the module.

I've also started on a more clear explanation in how to use this module, but that will need a bit more iterations to have it be readable and usable for new users of this module.

[cdepillabout]

This looks good, thanks for fixing it up!

[Vlix]

Ok, I've finished adding documentation and checking how the Haddocks look. Do tell if anything obvious stands out, or if something is confusing.

[cdepillabout]

I took another look at this and I think it looks pretty good!

Thanks again for fixing this up and getting it ready to merge in.

Also, please feel free to credit yourself in the ChangeLog.md, since you did a bunch of the work for this. (Although I guess thinking about this, I haven't been crediting myself in the ChangeLog, so I guess it makes sense for neither of us to credit ourselves!)

[Vlix]

TODO:

• Change InvalidPolicyReason's ProvidedLengths into Min-/MaximumLength
• Add TH function to easily validate a static policy at compile time

[Vlix]

Ok, added TemplateHaskell function and an extra InvalidPolicyReason.
Will go and add a TH function example to the documentation later (and then also update the ChangeLog.md).
If there's no blatant mistakes in the code/tests, I think we're finally done to update the package to 2.1.0.0

[Vlix]

Ok, I think I'm done with all the additions and documentation.
@cdepillabout If you could check the last couple of commits (or recheck the entire diff) for anything out of the ordinary, that'd be great.
I'll be doing a quick skim over the entire diff before merging after that.

[cdepillabout]

I took a look a this and it looks really good to me!