core: Add string-length.

* src/strings.c (string_length): New function.
* mes/module/mes/scm.mes (string-length): Remove.

mes/module/mes/scm.mes

@@ -194,9 +194,6 @@
 (define (make-string n . fill)
   (list->string (apply make-list n fill)))
 
-(define (string-length s)
-  (length (string->list s)))
-
 (define (string-ref s k)
   (list-ref (string->list s) k))
 

src/strings.c

@@ -260,3 +260,10 @@ string_append (SCM x) ///((arity . n))
     }
   return make_string (buf, size);
 }
+
+SCM
+string_length (SCM string)
+{
+  assert (TYPE (string) == TSTRING);
+  return MAKE_NUMBER (LENGTH (string));
+}