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/runtime.sx

@@ -366,6 +366,16 @@
      let printf fmt = print_string fmt
    end ;;
 
+   module Buffer = struct
+     let create _ = ref []
+     let add_string b s = b := s :: !b
+     let add_char b c = b := c :: !b
+     let contents b = String.concat \"\" (List.rev !b)
+     let length b = String.length (String.concat \"\" (List.rev !b))
+     let clear b = b := []
+     let reset = clear
+   end ;;
+
    module Sys = struct
      let os_type = \"SX\"
      let word_size = 64