`instantiate` should be replaced with `make-object` or `new` when feasible

[jackfirth]

#lang resyntax/test

test: "insantiate without positional arguments refactorable to new"
--------------------
#lang racket
(define (f cls x y)
  (instantiate cls () ([x x] [y y])))
--------------------
--------------------
#lang racket
(define (f cls x y)
  (new cls ([x x] [y y])))
--------------------

When instantiate is used without positional arguments, it's equivalent to new. When used without by-name arguments, it's equivalent to make-object. Mixing the two is complicated and uses of instantiate that don't need to mix them should be refactored to one of the two simpler object creation forms.