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).

cmd/wcc/ast.c

@@ -81,6 +81,8 @@ nkname(Nkind k)
 	case N_TRYUNW:		return "tryunw";
 	case N_MLET:		return "mlet";
 	case N_MASSIGN:		return "massign";
+	case N_TYPETEST:	return "typetest";
+	case N_TYPEASSERT:	return "typeassert";
 	case N_LAST:		return "last";
 	}
 	return "?";