ocaml: phase 4 's.[i]' string indexing syntax (+3 tests, 484 total)

parse-atom-postfix now dispatches three cases after consuming '.':

  .field  -> existing field/module access
  .(EXPR) -> existing local-open
  .[EXPR] -> new string-get syntax  (this commit)

Eval reduces (:string-get S I) to host (nth S I), which already returns
a one-character string for OCaml's char model.

Lets us write idiomatic OCaml string traversal:

  let s = "hi" in
  let n = ref 0 in
  for i = 0 to String.length s - 1 do
    n := !n + Char.code s.[i]
  done;
  !n  (* = 209 *)

lib/ocaml/parser.sx

@@ -571,6 +571,14 @@
                                   (consume! "op" ")")
                                   (set! head (list :let-open head inner))
                                   (loop)))))
+                          ((at-op? "[")
+                            (begin
+                              (advance-tok!)
+                              (let ((idx-expr (parse-expr)))
+                                (begin
+                                  (consume! "op" "]")
+                                  (set! head (list :string-get head idx-expr))
+                                  (loop)))))
                           (else
                             (let ((tok (peek-tok)))
                               (begin