ocaml: parser accepts top-level tuple patterns in match cases

Real OCaml accepts `match e1, e2 with | p1, p2 -> …` without
surrounding parens. parse-pattern previously stopped at the cons
layer (`p :: rest`) and treated a trailing `,` as a separator
the outer caller couldn't handle, surfacing as
"expected op -> got op ,".

Fix: `parse-pattern` now collects comma-separated patterns into a
:ptuple after parse-pattern-cons, before the optional `as` alias.
The scrutinee side already built tuples via parse-tuple, so both
sides are now symmetric.

lru_cache.ml (iter 258) reverts its workaround back to the natural
form:

  let rec take n lst = match n, lst with
    | 0, _ -> []
    | _, [] -> []
    | _, h :: r -> h :: take (n - 1) r

607/607 regressions clean.

lib/ocaml/baseline/lru_cache.ml

@@ -27,11 +27,10 @@ let put k v =
   let cleaned = remove !cache in
   let trimmed =
     if List.length cleaned >= cap then
-      let rec take n lst =
-        if n = 0 then []
-        else match lst with
-          | [] -> []
-          | h :: r -> h :: take (n - 1) r
+      let rec take n lst = match n, lst with
+        | 0, _ -> []
+        | _, [] -> []
+        | _, h :: r -> h :: take (n - 1) r
       in
       take (cap - 1) cleaned
     else cleaned