Allow declare forms in typeclass instances

[ornamentist]

Currently, we make a typeclass with functions identified by name:

(define-class (MyEq :a)
  (is-eq (:a -> :a -> Boolean)
  ; etc...

Sometime later, perhaps in a different file, we create a typeclass instance with functions identified by define forms:

(define-instance (MyEq Integer)
  (define (is-eq x y)
    ; etc...

Would it be possible to have an optional declare form with each typeclass instance function? I've found these very useful in Haskell to remind me of the type signature of the instance method, in terms of the instance:

(define-instance (MyEq Integer)
  (declare is-eq (Integer -> Integer -> Boolean))
  (define (is-eq x y)
    ; etc...

These declarations also help as a checkable docstring for each instance function.