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/eval.sx

@@ -557,6 +557,11 @@
                 ((dict? target) (get target fname))
                 (else (error
                   (str "ocaml-eval: not a record/module on .field: " target)))))))
+        ((= tag "string-get")
+          ;; (:string-get S I) — evaluate s.[i] as a char access.
+          (let ((s (ocaml-eval (nth ast 1) env))
+                (i (ocaml-eval (nth ast 2) env)))
+            (nth s i)))
         ((= tag "for")
           ;; (:for NAME LO HI DIR BODY) — DIR is "ascend" or "descend".
           (let ((name (nth ast 1))