More abstract function pointers

[kondrak]

Currently we can only use fn(f64, f64) -> f64 to store a function pointer, original library can work with anything (a Function struct with all possible variants included inside?)

[kondrak]

struct constructors could come in multiple variants for each function type (taking a pointer as an arg) and the function pointers themselves could/should be an Option<> value with each struct instance having only one of them set to Some()thing.