Add map-cons

This idiom of converting a list (L1 L2) of two lists into a list of
cons cells was appearing in a few places, so add a function to
encapsulate it.

ffmpeg-myth.scm

@@ -115,6 +115,8 @@
   (dsv-splitter (delete "" (dsv-splitter s #\newline)) ffprobe-separator))
 (define (k-v-parser s)
   (read-from-string (dsv-splitter s #\=)))
+(define (map-cons l)
+  (apply (lambda(x y) (map cons x y)) l))
 (define (make-dotted-alist s)
   (cond ((and (list? s) (= 2 (length s)) (symbol? (car s)))
 	 (cons (car s) (cadr s)))