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/parser.sx

@@ -49,6 +49,7 @@
   ocaml-op-table
   (list
     (list ":=" 1 :right)
+    (list "<-" 1 :right)
     (list "||" 2 :right)
     (list "or" 2 :right)
     (list "&&" 3 :right)