compat: Add 'exact-integer?' for Mes.

* gash/compat.scm [mes] (exact-integer?): New procedure.

gash/compat.scm

@@ -103,7 +103,8 @@
           call-with-input-string
           thunk?
           EXIT_SUCCESS
-          EXIT_FAILURE)
+          EXIT_FAILURE
+          exact-integer?)
 
   (define-macro (define-inlinable . rest)
     `(define ,@rest))
@@ -191,4 +192,7 @@
            (or (null? args) (symbol? args)))))
 
   (define EXIT_SUCCESS 0)
-  (define EXIT_FAILURE 1))
+  (define EXIT_FAILURE 1)
+
+  ;; Mes only has exact integers.
+  (define exact-integer? integer?))