ocaml: phase 6 Hashtbl.keys/values/bindings/remove/clear (+4 tests, 545 total)

Two new host primitives:
  _hashtbl_remove t k   -> dissoc the key from the underlying dict
  _hashtbl_clear  t     -> reset the cell to {}

Eight new OCaml-syntax helpers in runtime.sx Hashtbl module:
  bindings t            = _hashtbl_to_list t
  keys t                = List.map (fun (k, _) -> k) (...)
  values t              = List.map (fun (_, v) -> v) (...)
  to_seq t              = bindings t
  to_seq_keys / to_seq_values
  remove / clear / reset

The keys/values implementations use a 'fun pair -> match pair with
(k, _) -> k' indirection because parse-fun does not currently allow
tuple patterns directly on parameters. Same restriction we worked
around in iteration 98's let-pattern desugaring.

Also: a detour attempting to add top-level 'let (a, b) = expr'
support was started but reverted — parse-decl-let in the outer
ocaml-parse-program scope does not have access to parse-pattern
(which is local to ocaml-parse). Will need a slice + re-parse trick
later.

lib/ocaml/test.sh

@@ -1350,6 +1350,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5092)
 (eval "(ocaml-run \"let pair = (5, 7) in let (x, y) = pair in x * y\")")
 
+;; ── Hashtbl.keys / values / remove / clear ────────────────────
+(epoch 5100)
+(eval "(ocaml-run \"let t = Hashtbl.create 4 in Hashtbl.add t \\\"a\\\" 1; Hashtbl.add t \\\"b\\\" 2; List.length (Hashtbl.keys t)\")")
+(epoch 5101)
+(eval "(ocaml-run \"let t = Hashtbl.create 4 in Hashtbl.add t \\\"a\\\" 5; Hashtbl.add t \\\"b\\\" 7; List.fold_left (+) 0 (Hashtbl.values t)\")")
+(epoch 5102)
+(eval "(ocaml-run \"let t = Hashtbl.create 4 in Hashtbl.add t \\\"x\\\" 1; Hashtbl.add t \\\"y\\\" 2; Hashtbl.remove t \\\"x\\\"; Hashtbl.length t\")")
+(epoch 5103)
+(eval "(ocaml-run \"let t = Hashtbl.create 4 in Hashtbl.add t \\\"a\\\" 1; Hashtbl.clear t; Hashtbl.length t\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2145,6 +2155,12 @@ 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'
 
+# ── Hashtbl.keys/values/remove/clear ────────────────────────────
+check 5100 "Hashtbl.keys length"          '2'
+check 5101 "Hashtbl.values sum"           '12'
+check 5102 "Hashtbl.remove + length"      '1'
+check 5103 "Hashtbl.clear + length"       '0'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"