Five tags from ref/hare/errors/common.ha — invalid, noaccess, noentry,
exists, unsupported — all !void. lib/io keeps eof/closed as plain void
(no Hare analogue for ww's singleton-style done) and adds underread.
Drops errors.equal/isnil and the old str-sentinel surface.
Both used the -1 sentinel return; both had no external callers, so
the graduation is purely the API-shape change. utf8.runesz uses void
for "rune outside legal range"; bufio.readbyte uses void for EOF
(empty buffer). The full Hare shapes ((size | invalid) and
(u8 | EOF | io::error)) are still richer than this — those richer
returns arrive when utf8 grows an explicit invalid type and bufio
wires through io::stream's error path.
`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).
