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).

lib/errors/errors.ww

@@ -16,10 +16,10 @@ export fn isnil(e: error) bool = {
 	return e.len == 0;
 };
 
-// is — compare an error against a sentinel (or any other error). Pure
-// byte equality; same shape as strings.equal but kept here so callers
-// don't have to pull in strings just to compare.
-export fn is(e: error, want: error) bool = {
+// equal — compare an error against a sentinel (or any other error).
+// Pure byte equality. (Was `errors.is` before `is` became a keyword
+// for tagged-union type-tests; rename matches bytes.equal / strings.equal.)
+export fn equal(e: error, want: error) bool = {
 	if (e.len != want.len) { return false; };
 	let i: i32 = 0;
 	for (i < e.len) {