bus: rearrange bus-message.c function order to keep read and write calls together
bus: rely on explicit eof check instead of checking when mapping payload

This allows us to drop quite a bit of code.
bus: suppress creating empty parts in messages
bus: handler empty messages correctly when using gvariant marshalling
bus: properly deserialize gvariant fixed size arrays
bus: fix signature handling when exiting container
bus: fix rewinding in gvariant messages
bus: only accept gvariant native endian messages via kdbus
bus: beef up zero copy test case
update TODO