ocaml: phase 2 function | pat -> body (+4 tests, 198 total)

Sugar for fun + match. AST (:function CLAUSES) -> unary closure that
runs ocaml-match-clauses on its arg. let rec recognises :function as a
recursive rhs and ties the knot via cell, so

  let rec map f = function | [] -> [] | h::t -> f h :: map f t

works. ocaml-match-eval refactored to share clause-walk with function.

lib/ocaml/test.sh

@@ -507,6 +507,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 624)
 (eval "(ocaml-run \"let p = ref 1 in for i = 1 to 5 do p := !p * i done; !p\")")
 
+;; ── function (sugar for fun + match) ───────────────────────────
+(epoch 640)
+(eval "(ocaml-run \"(function | None -> 0 | Some x -> x) (Some 7)\")")
+(epoch 641)
+(eval "(ocaml-run \"let f = function | None -> 0 | Some x -> x in f None\")")
+(epoch 642)
+(eval "(ocaml-run \"let rec len = function | [] -> 0 | _ :: t -> 1 + len t in len [1; 2; 3]\")")
+(epoch 643)
+(eval "(ocaml-run-program \"let rec map f = function | [] -> [] | h :: t -> f h :: map f t;; map (fun x -> x * x) [1; 2; 3; 4]\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 60 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -808,6 +818,12 @@ check 622 "while loop"                   '15'
 check 623 "for 1..100 sum"               '5050'
 check 624 "for 1..5 product = 120"       '120'
 
+# ── function ────────────────────────────────────────────────────
+check 640 "function None|Some Some 7"    '7'
+check 641 "function None=0"              '0'
+check 642 "rec function len"             '3'
+check 643 "rec function map x*x"         '(1 4 9 16)'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"