matchable: Add conversion pattern(s)

[Moritz Heidkamp]

The attached patch adds two new pattern types to matchable:

(-> convert pat-1 … pat-n)
(-?> convert pat-1 … pat-n)

These patterns work like the (? predicate pat-1 … pat-n) pattern
in that convert must be an expression evaluating to a single
argument function which is applied to the corresponding
value. However, in contrast to ?, the following patterns
pat-1 … pat-n are applied to the value _returned_ by
convert. In addition, -?> only matches when convert
does not return #f, so it can be expressed as combination of
-> and ? like this:

(-> convert (? (lambda (x) x) pat-1 … pat-n))

The rationale is that it allows to match against a converted value and
at the same time being able to bind that value to a variable instead
of having to apply the conversion again in the body. One might argue
that -?> is not really necessary so I'd be happy with only
adding ->, too. It should be noted that this patch is not
backwards compatible.

Some examples:

(match '("foo" "10")
  (("foo" (-> string->number x)) (+ x x)))

;; => 20


(match '("foo" "bar")
  (("foo" (-> string->number x)) (+ x x)))

;; Error: (+) bad argument type: #f


(match '("foo" "10")
  (("foo" (-?> string->number x)) (+ x x))
  (else 'nothing))

;; => 20


(match '("foo" "bar")
  (("foo" (-?> string->number x)) (+ x x))
  (else 'nothing))

;; => nothing