ww: add Hare-style is/as postfix ops on tagged unions

`e is T` returns bool (variant tag == T's index); `e as T` unwraps
to T or exit(1) on mismatch. Postfix, same precedence as `:` cast.
TK_IS / N_TYPETEST / N_TYPEASSERT appended at the tail of their
enums so every prior numeric value stays unchanged — the
990_selfhost wwdump-diff stays byte-clean.

Cgen mirrors the match-case slot-based load (tag at +0, value at
+8/+16), so an N_IDENT tagged-union local works just like a
match scrutinee. Selfhost cgen inlines the slot resolution
because the wwstage cgen drops sign bits on `*i32` output
parameters in this position.

Renames `errors.is` -> `errors.equal` (the only naming collision;
the existing comment already noted it shared shape with
strings.equal/bytes.equal).

test/wcc/200_parse.c

@@ -162,6 +162,11 @@ main(void)
 		"fn try() (i32, str) = { return 0, \"\"; };",
 		"fn use_tuple() void = { let q, r = divmod(10, 3); };",
 		"fn assign_tuple() void = { q, r = divmod(10, 3); };",
+
+		/* Hare-style type test / type assertion (postfix) */
+		"fn ti(r: (i64 | i32)) bool = { return r is i64; };",
+		"fn ai(r: (i64 | i32)) i64 = { return r as i64; };",
+		"fn br(r: (i64 | str)) i32 = { if (r is i64) { return 1; }; return 0; };",
 	};
 	int n = sizeof parses / sizeof parses[0];
 	for (int i = 0; i < n; i++) {