wip: Add the basics for the Double Ratchet

lib/src/helpers.dart

@@ -0,0 +1,15 @@
+/// Flattens [inputs] and concatenates the elements.
+List<int> concat(List<List<int>> inputs) {
+  final tmp = List<int>.empty(growable: true);
+  for (final input in inputs) {
+    tmp.addAll(input);
+  }
+
+  return tmp;
+}
+
+List<int> pkcs7padding(List<int> input, int size) {
+  final paddingLength = size - input.length % size;
+  final padding = List<int>.filled(paddingLength, 0x0);
+  return concat([input, padding]);
+}