Multibase: Reuse part of the oscoin's code

[adinapoli-mndc]

This stemmed from a conversation with @kim . Currently in oscoin we have a BaseN.hs module which is quite pleasant as it uses data kinds to provide type safe base conversions, something that (from what I could see by skimming this library) it's a bit more verbose in the current implementation.

Ideally we should pull the oscoin code into this repo and finally switch oscoin to be using this library.

Have I synthesised things correctly?

[kim]

Yep, #9 :)

[adinapoli-mndc]

Oh, so my ticket is actually a dupe? Sorry 'bout that! 😅

[adinapoli-mndc]

Closed by #50