refactor and cleanup [skip ci]

submodules/Postbox/Sources/SimpleSet.swift

@@ -0,0 +1,23 @@
+import Foundation
+
+public struct SimpleSet<T: Equatable> {
+    private var items: [T] = []
+    
+    public init() {
+    }
+    
+    public mutating func insert(_ item: T) {
+        if !self.contains(item) {
+            self.items.append(item)
+        }
+    }
+    
+    public func contains(_ item: T) -> Bool {
+        for currentItem in self.items {
+            if currentItem == item {
+                return true
+            }
+        }
+        return false
+    }
+}