ocaml: phase 6 Either module + Hashtbl.copy (+4 tests, 602 total)

Either module (mirrors OCaml 4.12+ stdlib):
  left x / right x
  is_left / is_right
  find_left / find_right (return Option)
  map_left / map_right (single-side mappers)
  fold lf rf e            (case dispatch)
  equal eq_l eq_r a b
  compare cmp_l cmp_r a b (Left < Right)

Constructors are bare 'Left x' / 'Right x' (OCaml 4.12+ exposes them
directly without an explicit type-decl).

Hashtbl.copy:
  build a fresh cell with _hashtbl_create
  walk _hashtbl_to_list and re-add each (k, v)
  mutating one copy doesn't touch the other
  (Hashtbl.length t + Hashtbl.length t2 = 3 after fork-and-add
   verifies that adds to t2 don't appear in t)

lib/ocaml/runtime.sx

@@ -1030,6 +1030,48 @@
      let remove t k = _hashtbl_remove t k
      let reset t = _hashtbl_clear t
      let clear t = _hashtbl_clear t
+
+     let copy t =
+       let t' = _hashtbl_create 8 in
+       List.iter
+         (fun (k, v) -> _hashtbl_add t' k v)
+         (_hashtbl_to_list t);
+       t'
+   end ;;
+
+   module Either = struct
+     let left x = Left x
+     let right x = Right x
+
+     let is_left e = match e with Left _ -> true | Right _ -> false
+     let is_right e = match e with Left _ -> false | Right _ -> true
+
+     let find_left e =
+       match e with Left x -> Some x | Right _ -> None
+     let find_right e =
+       match e with Left _ -> None | Right x -> Some x
+
+     let map_left f e =
+       match e with Left x -> Left (f x) | Right x -> Right x
+     let map_right f e =
+       match e with Left x -> Left x | Right x -> Right (f x)
+
+     let fold lf rf e =
+       match e with Left x -> lf x | Right x -> rf x
+
+     let equal eq_l eq_r a b =
+       match a with
+       | Left x ->
+         (match b with Left y -> eq_l x y | Right _ -> false)
+       | Right x ->
+         (match b with Left _ -> false | Right y -> eq_r x y)
+
+     let compare cmp_l cmp_r a b =
+       match a with
+       | Left x ->
+         (match b with Left y -> cmp_l x y | Right _ -> -1)
+       | Right x ->
+         (match b with Left _ -> 1 | Right y -> cmp_r x y)
    end ;;
 
    module Map = struct