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/ProfileModel.swift

@@ -14,14 +14,14 @@ class ProfileModel: ObservableObject, Equatable {
     @Published var progress: Int = 0
     
     var events: EventHolder
-    let pubkey: String
+    let pubkey: Pubkey
     let damus: DamusState
     
-    var seen_event: Set<String> = Set()
+    var seen_event: Set<NoteId> = Set()
     var sub_id = UUID().description
     var prof_subid = UUID().description
     
-    init(pubkey: String, damus: DamusState) {
+    init(pubkey: Pubkey, damus: DamusState) {
         self.pubkey = pubkey
         self.damus = damus
         self.events = EventHolder(on_queue: { ev in
@@ -29,7 +29,7 @@ class ProfileModel: ObservableObject, Equatable {
         })
     }
     
-    func follows(pubkey: String) -> Bool {
+    func follows(pubkey: Pubkey) -> Bool {
         guard let contacts = self.contacts else {
             return false
         }