Add the FVect datatype

[mattpolzin]

This adds the Data.FVect (fin-based vect) datatype as proposed in #16. It also adds Data.FVect.Capacity which is a view into an FVect that makes it easy to add to an FVect if it has any capacity (or prove it is full otherwise).

Please let me know if you would like to see any changes, additions, or subtractions from the included modules.

[Matthew-Mosior]

Really excited to have this in the codebase, this lgtm!

@stefan-hoeck

Any changes you'd like to see?

[stefan-hoeck]

Thanks for this addition. Out of curiosity: Can you give an example when you found this data structure to be useful?

[mattpolzin]

Can you give an example when you found this data structure to be useful?

I looked around and I don't have any long term projects that use it so although I've used it for several explorations in the past I cannot recall exactly how it was used. We'll need to settle for the abstract conceptual need "I know that when I perform a filtering operation on a vect, its length is less than or equal to its length prior to the operation -- I want to structurally encode that property instead of proving it ad-hoc." That was my original motivation.