Introduce an enum to represent different functions

[tgross35]

Next steps so I don't forget:

• On the enum add functions for string names
• Make a single mod acos { pub struct Func; } within func for each function. This could just be a type, we use modules so we don't need to convert casing
• Make a trait for input and output types, name, base float type, etc. This should replace everything that the macro has to pass around all the time.
• Other traits can extend this, e.g. MpOp will say how to use Rug with that type

I think this should reduce how much we need to rely on proc macro magic