ocaml: phase 2 mutable record fields r.f <- v (+4 tests, 451 total)

<- added to op-table at level 1 (same as :=). Eval short-circuits on
<- to mutate the lhs's field via host SX dict-set!. The lhs must be a
:field expression; otherwise raises.

Tested:
  let r = { x = 1; y = 2 } in r.x <- 5; r.x       (5)
  let r = { x = 0 } in for i = 1 to 5 do r.x <- r.x + i done; r.x  (15)
  let r = { name = ...; age = 30 } in r.name <- "Alice"; r.name

The 'mutable' keyword in record type decls is parsed-and-discarded;
runtime semantics: every field is mutable. Phase 2 closes this gap
without changing the dict-based record representation.

lib/ocaml/eval.sx

@@ -442,6 +442,18 @@
                 (let ((cell (ocaml-eval (nth ast 2) env))
                       (new-val (ocaml-eval (nth ast 3) env)))
                   (begin (set-nth! cell 0 new-val) nil)))
+              ;; <- mutates a record field. The lhs must be a (:field ...).
+              ((= op "<-")
+                (let ((lhs-ast (nth ast 2)) (new-val (ocaml-eval (nth ast 3) env)))
+                  (cond
+                    ((= (ocaml-tag-of lhs-ast) "field")
+                      (let ((target (ocaml-eval (nth lhs-ast 1) env))
+                            (fname (nth lhs-ast 2)))
+                        (begin (dict-set! target fname new-val) nil)))
+                    (else
+                      (error
+                        (str "ocaml-eval: <- expects a field-access lhs, got "
+                             (ocaml-tag-of lhs-ast)))))))
               (else
                 (ocaml-eval-op op
                   (ocaml-eval (nth ast 2) env)