Pre-allocate the full `Vec` prior to serializing as a `Vec<u8>`

Pre-allocate the full `Vec` prior to serializing as a `Vec<u8>`

We end up generating a substantial amount of allocations just
doubling `Vec`s when serializing to them, and our
`serialized_length` method is generally rather effecient, so we
just rely on it and allocate correctly up front.