First Class Functions

[Checkmate50]

To support TypeScript, it would be helpful to upgrade our functions to first-class so they can be passed as function arguments. Types of functions should probably be a->b or fun<a, b> depending on how fancy we want to be. Types should be infer-able and follow the subtyping structure of the usual FL functions (a < a', b' < b => a->b < a'->b').