src: simplify UnionBytes

Before this commit it was using a tagged union to store the one-byte and
two-byte pointers.

From a `sizeof(UnionBytes)` perspective that makes no difference - there
is a hole between the tag and the union - and it makes the code just a
little harder to reason about, IMO.

PR-URL: https://github.com/nodejs/node/pull/29116


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>