Support Function Overloading

[DavePearce]

The corset language supports a fairly primtiive form of function overloading, which needs to be implemented. For example on arity:

(defpurefun (if-zero cond then) (if (vanishes! cond) then))
(defpurefun (if-zero cond then else) (if (vanishes! cond) then else))

Also, on parameter types:

(defpurefun ((eq :binary@bool :force) (x :binary) (y :binary)) (^ (- x y) 2))
(defpurefun ((eq :binary@bool :force) x y) (- 1 (~ (eq! x y))))