ocaml: phase 4 'arr.(i)' and 'arr.(i) <- v' array indexing (+3 tests, 515 total)

parse-atom-postfix's '.()' branch now disambiguates between let-open
and array-get based on whether the head is a module path (':con' or
':field' chain rooted in ':con'). Module paths still emit
(:let-open M EXPR); everything else emits (:array-get ARR I).

Eval handles :array-get by reading the cell's underlying list at
index. The '<-' assignment handler now also accepts :array-get lhs
and rewrites the cell with one position changed.

Idiomatic OCaml array code now works:

  let a = Array.make 5 0 in
  for i = 0 to 4 do a.(i) <- i * i done;
  a.(3) + a.(4)               = 25

  let a = Array.init 4 (fun i -> i + 1) in
  a.(0) + a.(1) + a.(2) + a.(3)  = 10

  List.(length [1;2;3])         = 3   (* unchanged: List is a module *)

lib/ocaml/eval.sx

@@ -490,6 +490,22 @@
                       (let ((target (ocaml-eval (nth lhs-ast 1) env))
                             (fname (nth lhs-ast 2)))
                         (begin (dict-set! target fname new-val) nil)))
+                    ((= (ocaml-tag-of lhs-ast) "array-get")
+                      ;; (:array-get ARR I) <- v : rewrite the underlying
+                      ;; list in the ref cell with one cell changed.
+                      (let ((arr (ocaml-eval (nth lhs-ast 1) env))
+                            (i (ocaml-eval (nth lhs-ast 2) env)))
+                        (begin
+                          (define replace
+                            (fn (lst k)
+                              (cond
+                                ((= lst (list)) (list))
+                                ((= k 0) (cons new-val (rest lst)))
+                                (else
+                                  (cons (first lst)
+                                        (replace (rest lst) (- k 1)))))))
+                          (set-nth! arr 0 (replace (nth arr 0) i))
+                          nil)))
                     (else
                       (error
                         (str "ocaml-eval: <- expects a field-access lhs, got "
@@ -602,6 +618,11 @@
           (let ((s (ocaml-eval (nth ast 1) env))
                 (i (ocaml-eval (nth ast 2) env)))
             (nth s i)))
+        ((= tag "array-get")
+          ;; (:array-get ARR I) — Array.get on a ref-of-list.
+          (let ((arr (ocaml-eval (nth ast 1) env))
+                (i (ocaml-eval (nth ast 2) env)))
+            (nth (nth arr 0) i)))
         ((= tag "for")
           ;; (:for NAME LO HI DIR BODY) — DIR is "ascend" or "descend".
           (let ((name (nth ast 1))