Add partial and partialr

[AlexKnauth]

partial and partialr are sort of like curry and curryr, but much simpler. They only go "one level down." Unlike curry, the behavior doesn't change when you reduce or widen the procedure's arity (unless the arity causes an error), and partial and partialr fully support keyword arguments.

[jackfirth]

I think this belongs in the arguments package, which has a stronger focus on manipulating keyword arguments. Using that package could make the implementation simpler too:

;; arity fixups omitted for brevity
(define/arguments (partial f+args)
  (define f (first (arguments-positional f+args)))
  (define args
    (make-arguments (rest (arguments-positional f+args))
                    (arguments-keyword f+args)))
  (partial/arguments f args))

(define (partial/arguments f partial-args)
  (lambda/arguments args (apply/arguments f (arguments-merge partial-args args))))

This implementation allows overwriting keywords which may or may not be a good idea. For example, ((partial draw #:color 'red) pic #:color 'blue) is equivalent to (draw pic #:color 'blue). Should that raise an arity exception instead?