Make GetSerializeSize a wrapper on top of CSizeComputer

Given that in default GetSerializeSize implementations created by
ADD_SERIALIZE_METHODS we're already using CSizeComputer(), get rid
of the specialized GetSerializeSize methods everywhere, and just use
CSizeComputer. This removes a lot of code which isn't actually used
anywhere.

For CCompactSize and CVarInt this actually removes a more efficient
size computing algorithm, which is brought back in a later commit.