Implement a nominal version of the library.

[lambdageek]

In unbound Unbound.Nominal there is (the beginnings of) a version of the library that doesn't use a locally nameless representation, instead it just uses a simple nominal representation. @sweirich says that the idea was to investigate whether this version would perform better, but that the code is not worth porting as is.

If we decide to do this, one thing I'd look into is whether we can avoid having two completely unrelated Alpha type classes (harder), two different sets of binding combinators (easier) and even whether it's possible to mix named and nameless representations for two different sorts of names in a single project.