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/bufio/bufio.ww

@@ -52,7 +52,7 @@ export fn avail(b: *buf) i32 = {
 
 // Distinct alias so `(str | linerr)` has two variant types the
 // tagged-union machinery can keep apart at the tag level. The error
-// variant carries a short description; callers compare with errors.is
+// variant carries a short description; callers compare with errors.equal
 // or just inspect by length.
 type linerr = str;