The operator<< / operator>> templates for MessageConst / MessageMutable
added in 4d8b7430 have no users in the tree (libs, tests, utils, main).
Drop them along with the pibinarystream.h include they required; the
binary-stream fallback static_assert in pibinarystream.h already gives a
clear compile error if external code ever streams an MQTT message.
- Rename __PIP_TYPENAME_DECLARE to PIP_REGISTER_TYPENAME across all headers
- Add PIP_DECLARE_TYPENAME(name) macro for in-class type name declaration
- Replace all static const char * __pip_typename__() member functions with PIP_DECLARE_TYPENAME