Handle multiple variables in `define-let-to-double-define`

[jackfirth]

The define-let-to-double-define rule can rewrite this:

(define a
  (let ([b 1])
    (+ b 10)))

to this:

(define b 1)
(define a (+ b 10))

But it can't rewrite this:

(define a
  (let ([b 1] [c 2])
    (+ b c 10)))

to this:

(define b 1)
(define c 2)
(define a (+ a b 10))

But that's a valuable refactoring to do, as seen here. The rule should also be renamed to define-let-to-multi-define since this could produce several definitions.

[jackfirth]

Blocked on #641.