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/Views/QRCodeView.swift

@@ -9,7 +9,7 @@ import SwiftUI
 import CoreImage.CIFilterBuiltins
 
 struct ProfileScanResult: Equatable {
-    let pubkey: String
+    let pubkey: Pubkey
     
     init(hex: String) {
         self.pubkey = hex
@@ -42,7 +42,7 @@ struct ProfileScanResult: Equatable {
 
 struct QRCodeView: View {
     let damus_state: DamusState
-    @State var pubkey: String
+    @State var pubkey: Pubkey
     
     @Environment(\.presentationMode) var presentationMode