ocaml: phase 4 'let PATTERN = expr in body' tuple destructuring (+3 tests, 541 total)

When 'let' is followed by '(', parse-let now reads a full pattern
(via the existing parse-pattern used by match), expects '=', then
'in', and desugars to:

  let PATTERN = EXPR in BODY  =>  match EXPR with PATTERN -> BODY

This reuses the entire pattern-matching machinery, so any pattern
the match parser accepts works here too — paren-tuples, nested
tuples, cons patterns, list patterns. No 'rec' allowed for pattern
bindings (real OCaml's restriction).

  let (a, b) = (1, 2) in a + b              = 3
  let (a, b, c) = (10, 20, 30) in a + b + c = 60
  let pair = (5, 7) in
  let (x, y) = pair in x * y                = 35

Also retroactively cleaned up Printf's iter-97 width-pos packing
hack ('width * 1000000 + spec_pos') — it's now
'let (width, spec_pos) = parse_width_loop after_flags in ...' like
real OCaml.

lib/ocaml/test.sh

@@ -1342,6 +1342,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5084)
 (eval "(ocaml-run \"Printf.sprintf \\\"hi=%-3d, hex=%04x\\\" 9 15\")")
 
+;; ── let (a, b) = expr in body — tuple destructure ─────────────
+(epoch 5090)
+(eval "(ocaml-run \"let (a, b) = (1, 2) in a + b\")")
+(epoch 5091)
+(eval "(ocaml-run \"let (a, b, c) = (10, 20, 30) in a + b + c\")")
+(epoch 5092)
+(eval "(ocaml-run \"let pair = (5, 7) in let (x, y) = pair in x * y\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2132,6 +2140,11 @@ check 5082 "%05d 42 zero-pad"             '"00042"'
 check 5083 "%4s hi"                       '"  hi"'
 check 5084 "%-3d %04x mixed"              '"hi=9  , hex=000f"'
 
+# ── let (a, b) = expr in body ───────────────────────────────────
+check 5090 "let (a, b) = (1,2)"           '3'
+check 5091 "let (a, b, c) = (10,20,30)"   '60'
+check 5092 "let pair; let (x, y) = pair"  '35'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"