mes: Add string-map.

* module/srfi/srfi-13.mes (string-map): New function.
* tests/srfi-13.test ("string-map"): Test it.

module/srfi/srfi-13.mes

@@ -154,3 +154,6 @@
 (define (string-trim-both string . pred)
   ((compose string-trim string-trim-right) string))
 
+(define (string-map f string)
+  (list->string (map f (string->list string))))
+

tests/srfi-13.test

@@ -77,4 +77,7 @@ exit $?
 (pass-if-equal "string-trim-both" "foo"
              (string-trim-both "  foo  "))
 
+(pass-if-equal "string-map" "fuubar"
+             (string-map (lambda (c) (if (eq? c #\o) #\u c)) "foobar"))
+
 (result 'report)