Split D-Bus and GVariant ser/de

I realized very late how much incompatibility is there between D-Bus and GVariant serialization so ended up with a lot of format checks when I added GVariant format support. I'm creating this as a TODO to clean up the code by splitting the ser/de for each format after !161 (merged) is merged.

NOTE: The Serialize and Deserializer structs are part of public API.