ocaml: phase 2 references ref/!/:= (+6 tests, 189 total)

ref is a builtin boxing its arg in a one-element list. Prefix ! parses
to (:deref ...) and reads via (nth cell 0). := joins the binop
precedence table at level 1 right-assoc and mutates via set-nth!.
Closures share the underlying cell.

lib/ocaml/parser.sx

@@ -48,6 +48,7 @@
 (define
   ocaml-op-table
   (list
+    (list ":=" 1 :right)
     (list "||" 2 :right)
     (list "or" 2 :right)
     (list "&&" 3 :right)
@@ -401,6 +402,8 @@
             (cond
               ((at-op? "-")
                 (begin (advance-tok!) (list :neg (parse-prefix))))
+              ((at-op? "!")
+                (begin (advance-tok!) (list :deref (parse-prefix))))
               ((at-kw? "not")
                 (begin (advance-tok!) (list :not (parse-prefix))))
               (else (parse-app)))))