ndb: switch to nostrdb notes

This is a refactor of the codebase to use a more memory-efficient
representation of notes. It should also be much faster at decoding since
we're using a custom C json parser now.

Changelog-Changed: Improved memory usage and performance when processing events

damus/Models/PostBlock.swift

@@ -6,34 +6,3 @@
 //
 
 import Foundation
-
-enum PostBlock {
-    case text(String)
-    case ref(ReferencedId)
-    case hashtag(String)
-    
-    var is_text: String? {
-        if case .text(let txt) = self {
-            return txt
-        }
-        return nil
-    }
-    
-    var is_hashtag: String? {
-        if case .hashtag(let ht) = self {
-            return ht
-        }
-        return nil
-    }
-    
-    var is_ref: ReferencedId? {
-        if case .ref(let ref) = self {
-            return ref
-        }
-        return nil
-    }
-}
-
-func parse_post_textblock(str: String, from: Int, to: Int) -> PostBlock {
-    return .text(String(substring(str, start: from, end: to)))
-}