Extend `taffy-let` to work with `set!` and `match`, treating its body as a pattern

[rocketnia]

We could have a hyperbracketed operation which represents an expression that can be partially reversed, for use in set!.

Better yet, we could just reuse taffy-let for this.

In an expression context, these are already equivalent:

(taffy-let ([tail (hash-ref z 'key)])
  `(cons ,x (cons ,(car y) ,tail)))

(let ([tail (hash-ref z 'key)] [temp1 x] [temp2 (car y)])
  (cons temp1 (cons temp2 tail)))

In a set! left-hand side context, these would be equivalent:

(set!
  (taffy-let ([tail (hash-ref z 'key)])
    `(cons ,x (cons ,(car y) ,tail)))
  w)

(match w
  [ (cons temp1 (cons temp2 tail))
    (set! (hash-ref z 'key) tail)
    (set! x temp1)
    (set! (car y) temp2)])

The match-let syntax would even make sense in a match expander context. These would be equivalent there:

(match w
  [
    (taffy-let ([tail (vector a b)])
      `(cons ,x (cons ,(list y) ,tail)))
    (make-result a b x y)])

(match w
  [
    (app
      (match-lambda
        [ (cons temp1 (cons temp2 tail))
          (list tail temp1 temp2)]
        [_ #f])
     (list (vector a b) x (list y)))
    (make-result a b x y)])

Furthermore, when taffy-let is used as a set! transformer or a match expander, it might make sense to allow more than one clause in the taffy-let body.