compat: Add 'call-with-input-string' for Mes.

* gash/compat.scm [mes] (call-with-input-string): New procedure.

gash/compat.scm

@@ -100,6 +100,7 @@
           file-port?
           input-port?
           output-port?
+          call-with-input-string
           thunk?)
 
   (define-macro (define-inlinable . rest)
@@ -169,6 +170,13 @@
   (define (input-port? port) #f)
   (define (output-port? port) #f)
 
+  (define (call-with-input-string str proc)
+    (let ((port (open-input-string str)))
+      (call-with-values (lambda () (proc port))
+        (lambda results
+          (close-port port)
+          (apply values results)))))
+
   ;; Fix 'dup' interface.
   (let ((mes/dup dup))
     (define* (guile/dup fd #:optional new)