Refactor memory data

[emilk]

This refactors the widget state storage introduced by @optozorax in #257

• Unify the four buckets (data, data_temp, id_data and id_data_temp) into a single data.
  • Less complexity, and also less chance of error (storing in one bucket, reading from another).
• Store data by Id and TypeId.
  • Users can thus reuse the same Id to store many types.
  • Uses a simple xor of id and typeid, which is fast and good since both id and typeid are already high-entropy hashes.
• Use different suffixes on the functions to pick if you want the data persisted or not (get_temp, insert_persisted, etc).
  • Writing with one suffix and reading with the other works.
• To store state not bound to a specific Id (i.e. only based on type), use the new Id::null as the key.