implement closures

tests/modules/greet.sex

@@ -35,6 +35,19 @@
   (++ greet-count)
   (printf "hello, %s\n" name))
 
+;;; A closure type crossing the boundary. Both units generate the
+;;; struct for this signature independently, so they have to agree on
+;;; its tag and its layout, or the value is passed wrong and nothing
+;;; says so.
+(pub fn make-adder ((n int)) (closure ((int)) int)
+  (return (closure ((b int)) int (n)
+            (return (+ n b)))))
+
+;;; The other direction: a closure built by the importer, whose code
+;;; pointer is static in *its* object, called from here
+(pub fn apply-twice ((f (closure ((int)) int)) (x int)) int
+  (return (f (f x))))
+
 ;;; Not `pub': invisible to importers, and static in the generated C.
 (fn unused-helper () void
   (printf "private\n"))