refactor: add Pubkey, Privkey, NoteId string aliases

This is a non-behavioral change in preparation for the actual switchover
from Strings to Ids. The purpose of this kit is to reduce the size of
the switchover commit which is going to be very large.

damus/Models/DamusState.swift

@@ -50,7 +50,7 @@ struct DamusState {
         return stored
     }
     
-    var pubkey: String {
+    var pubkey: Pubkey {
         return keypair.pubkey
     }