Reorganize module structure

[jhelovuo]

The current module structure historically derives from the structure of DDS and RTPS specifications, which is not ideal for (this) implementation.

Planned changes:

• Split current dds module into two: dds for the application API (DDS spec) parts, and rtps for network protocol behaviour implementation (RTPS spec).
• discovery: rename topic_data into sedp (Simple Endpoint Discovery Protocol).
• messages: module structure is quite deep and complicated. Maybe simplify? Move this under rtps module.
• serialization: Move PL_CDR (de)serializer (incl. visitors.rs ) and builtin data (de)serializer under Discovery, because they are used only there. message.rs and submessage.rs should be under rtps module.
• structure: builtin_endoint belongs under Discovery. cache_change and dds_cache should be under rtps. paramter_id belongs under Discovery. rpc could perhaps be under dds.