Include function definitions

[zenna]

An AST should support function definitions.

This allows more compact definitions of code, better potential for symbolic reasoning, and also allows the potential of recursive functions.

It's not trivial because:

1. We have to somehow extract the function definition
2. Where should these function definitions "live"? In the definition of a particular function? For instance if we say some Y = myf(uniform(0,1)), where should the ast of myf be within Y?