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

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

src/core/matrix/pdu/state_key.rs

@@ -0,0 +1,8 @@
+use smallstr::SmallString;
+
+use super::ShortId;
+
+pub type StateKey = SmallString<[u8; INLINE_SIZE]>;
+pub type ShortStateKey = ShortId;
+
+const INLINE_SIZE: usize = 48;