Move parameter type casting to parse phase

[Cinders-P]

function_base was difficult to reason with and I didn't see the point of the least_offended considering we have no optional parameters (i.e. we're using overloading instead), so I've gone ahead and simplified it in #35. Still not entirely satisfied with how the dynamic types are happening right now: things like ints are only checked/casted at runtime.

Preferably, I would do argument casting in parse and leave only the function types-matching in evaluate. Leaving this placeholder issue here for that. Right now, we don't even retain the types of Image or Directory after casting (which only returns raw values), so this would require some refactoring.