Support multi-arity methods in defprotocol / reify / extend-type

[borkdude]

TODO:

• test for defining multi-arity protocol and implementing it with defrecord
• test for multi-arity multi-method defs

In defprotocol/reify/defrecord there's this syntax for multi-arity methods:

(deftype Foo [a b c]
  P
  (foo [x] a)
  (bar-me [x] b)
  (bar-me [x y] (+ c y)))

whereas extend-type has this:

(extend-type MyType
  Countable
    (cnt [c] ...)
  Foo
    (bar [x y] ...)
    (baz ([x] ...) ([x y zs] ...)))
What's the official syntax? (edited) 

Example:

user=> (defprotocol IFruit (subtotal [item] [item subtotal]))
IFruit
user=> (defrecord Apple [price] IFruit (subtotal [_] price) (subtotal [_ discount] (- price discount)))
user.Apple
user=> (subtotal (->Apple 100))
100
user=> (subtotal (->Apple 100) 5)
95

Also see #401