feat: introduce keyupdate messages informing contacts about relay changes

When the published relay list changes, key-contacts are informed with an
unsigned message carrying the re-signed key, encrypted to a chunk of contacts
at a time. It is shaped like a receipt notification naming no message, so that
cores which know nothing about keyupdates trash it as well.

See the src/keyupdate.rs module docs for the design.

src/lib.rs

@@ -72,6 +72,7 @@ pub mod ephemeral;
 mod imap;
 pub mod imex;
 pub mod key;
+mod keyupdate;
 pub mod location;
 pub mod login_param;
 pub mod message;