iter: make safer by using NdbNote instead of unsafe pointers

If we have an owned note, we could lose track of the lifetime and then
crash. Let's make sure we always have an NdbNote instead

nostrdb/NdbTagIterator.swift

@@ -8,9 +8,21 @@
 import Foundation
 
 struct TagSequence: Sequence {
-    let note: UnsafeMutablePointer<ndb_note>
+    let note: NdbNote
     let tag: UnsafeMutablePointer<ndb_tag>
 
+    var count: UInt16 {
+        tag.pointee.count
+    }
+
+    subscript(index: Int) -> NdbTagElem? {
+        if index >= tag.pointee.count {
+            return nil
+        }
+
+        return NdbTagElem(note: note, tag: tag, index: Int32(index))
+    }
+
     func makeIterator() -> TagIterator {
         return TagIterator(note: note, tag: tag)
     }
@@ -29,10 +41,14 @@ struct TagIterator: IteratorProtocol {
     }
 
     var index: Int32
-    let note: UnsafeMutablePointer<ndb_note>
+    let note: NdbNote
     var tag: UnsafeMutablePointer<ndb_tag>
 
-    init(note: UnsafeMutablePointer<ndb_note>, tag: UnsafeMutablePointer<ndb_tag>) {
+    var count: UInt16 {
+        tag.pointee.count
+    }
+
+    init(note: NdbNote, tag: UnsafeMutablePointer<ndb_tag>) {
         self.note = note
         self.tag = tag
         self.index = 0