Add implementation for Clojure-like multi-methods

[oubiwann]

Add implementation for Clojure-like multi-methods

[oubiwann]

Made some more progress on this tonight (see changes 7ea02cc and d218225). The following now works:

(defmulti area)
(defmulti perim)

(defmethod area 'triangle
  ((`#m(base ,b height ,h))
   (* b h (/ 1 2))))

(defmethod perim 'triangle
  ((`#m(base ,b height ,h))
   (+ b h (math:sqrt (+ (* b b)
                        (* h h))))))

(defmethod area 'circle
  ((`#m(diameter ,d))
    (area `#m(type circle radius ,(/ d 2))))
  ((`#m(radius ,r))
   (* (math:pi) r r)))

(defmethod perim 'circle
  ((`#m(diameter ,d))
    (perim `#m(type circle radius ,(/ d 2))))
  ((`#m(radius ,r))
    (* 2 (math:pi) r)))

With usage being simply this:

> (area #m(type triangle base 6 height 10))
30.0
> (perim #m(type triangle base 6 height 10))
27.6619037896906
> (area #m(type circle diameter 8))
50.26548245743669
> (perim #m(type circle diameter 8))
25.132741228718345

So far, the macros only support the rather trivial usage shown above, with the type hard-coded. The next steps are:

1. Allow users to pass the attribute name in defmulti
2. Support multiple attributes (for multiple dispatch)