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/tok.c

@@ -29,6 +29,7 @@ static const struct kwent kwtab[] = {
 	{ "fn",       TK_FN       },
 	{ "for",      TK_FOR      },
 	{ "if",       TK_IF       },
+	{ "is",       TK_IS       },
 	{ "let",      TK_LET      },
 	{ "match",    TK_MATCH    },
 	{ "nil",      TK_NIL      },
@@ -89,6 +90,7 @@ tokname(Tkind k)
 	case TK_TRUE:		return "true";
 	case TK_FALSE:		return "false";
 	case TK_AS:		return "as";
+	case TK_IS:		return "is";
 	case TK_STATIC:		return "static";
 	case TK_MATCH:		return "match";
 	case TK_CONST:		return "const";