mes: Add const.

* module/mes/scm.mes (const): New function.
* tests/scm.test ("const"): Test it.

module/mes/scm.mes

@@ -351,3 +351,7 @@
 (define (negate proc)
   (lambda args
     (not (apply proc args))))
+
+(define (const . rest)
+  (lambda (. _)
+    (car rest)))

tests/scm.test

@@ -159,4 +159,7 @@ exit $?
 (pass-if-equal "octal" 65 #o101)
 (pass-if-equal "hex" 257 #x101)
 
+(pass-if-equal "negate" #t ((negate eq?) 0 1))
+(pass-if-equal "const" 42 ((const 42) 1 2 3 4))
+
 (result 'report)