Add optimization pass

[madsobitsoe]

This is very much premature optimization, but here goes.

If one of the goals of this project is to produce teensy binaries, the parsed AST should most definitely be reduced to the smallest equivalent form we can achive.

Add (Const a, Const b) is equivalent to Const (a+b).
As long as the operation is associative, it should be possible to rewrite the AST.

Sub (e1, e2) can not be directly optimized, but the sub-expression (possibly) can. Recursively optimizing could potentially replace the Sub expression with a Const expression.