This allows a very compact notation for serialization of vectors whose
elements are not serialized using their default encoding.
This is a partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/18021 | PR18021]] [2/4]
https://github.com/bitcoin/bitcoin/pull/18021/commits/abf86243568af380c1384ac4e0bfcdcfd4dab085