ocaml: phase 1+6 Buffer + parser !x in app args (+3 tests, 425 total)

Parser fix: at-app-start? and parse-app's loop recognise prefix !
as a deref of the next app arg. So 'List.rev !b' parses as
'(:app List.rev (:deref b))' instead of stalling at !.

Buffer module backed by a ref holding string list:
  create _ = ref []
  add_string b s = b := s :: !b
  contents b = String.concat "" (List.rev !b)
  add_char/length/clear/reset

lib/ocaml/parser.sx

@@ -167,7 +167,7 @@
                 ((= tt "ctor") true)
                 ((and (= tt "keyword") (or (= tv "true") (= tv "false")))
                   true)
-                ((and (= tt "op") (or (= tv "(") (= tv "[") (= tv "{"))) true)
+                ((and (= tt "op") (or (= tv "(") (= tv "[") (= tv "{") (= tv "!"))) true)
                 (else false)))))
         (set!
           parse-pattern-atom
@@ -532,7 +532,7 @@
                 ((= tt "ctor") true)
                 ((and (= tt "keyword") (or (= tv "true") (= tv "false") (= tv "begin")))
                   true)
-                ((and (= tt "op") (or (= tv "(") (= tv "[") (= tv "{"))) true)
+                ((and (= tt "op") (or (= tv "(") (= tv "[") (= tv "{") (= tv "!"))) true)
                 (else false)))))
         (define parse-atom-postfix
           (fn ()
@@ -568,7 +568,12 @@
                     (when
                       (at-app-start?)
                       (let
-                        ((arg (parse-atom-postfix)))
+                        ((arg
+                          (cond
+                            ((at-op? "!")
+                              (begin (advance-tok!)
+                                     (list :deref (parse-atom-postfix))))
+                            (else (parse-atom-postfix)))))
                         (begin (set! head (list :app head arg)) (loop))))))
                 (loop)
                 head))))