move core::pdu and core::state_res into core::matrix::

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/matrix/mod.rs

@@ -0,0 +1,9 @@
+//! Core Matrix Library
+
+pub mod event;
+pub mod pdu;
+pub mod state_res;
+
+pub use event::Event;
+pub use pdu::{PduBuilder, PduCount, PduEvent, PduId, RawPduId, StateKey};
+pub use state_res::{EventTypeExt, RoomVersion, StateMap, TypeStateKey};