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/runtime.sx

@@ -562,7 +562,7 @@
              else cont := false
            else cont := false
          done;
-         (!w) * 1000000 + (!i)
+         (!w, !i)
        in
        let rec walk pos prefix =
          if pos >= n then prefix
@@ -573,9 +573,7 @@
              let left_flag = ref false in
              let zero_flag = ref false in
              let after_flags = parse_flags_loop (pos + 1) left_flag zero_flag in
-             let packed = parse_width_loop after_flags in
-             let width = packed / 1000000 in
-             let spec_pos = packed - width * 1000000 in
+             let (width, spec_pos) = parse_width_loop after_flags in
              if spec_pos < n && is_spec (_string_get fmt spec_pos) then
                let spec = _string_get fmt spec_pos in
                let left = !left_flag in