Don't bother typing the key to the value in `DelegatingKey`; we don't use it anyway.
Rename the `with` and `without` methods in the LimitedPropertyMap to `set` and `remove`.
Pull up `set` and `remove` to PropertyMap.
Move `LimitedPropertyMap::keys` to `AbstractFixedRekord`.
Move `LimitedPropertyMap::acceptedKeys` to `AbstractFixedRekord`.
Create a specific exception for an unacceptable key.
Use the PropertyMap interface.
Reorder methods.
Replace `Key::of` with `Key::set`.
Check the acceptability of keys in `ValidatingRekord`.
Delay construction of the PersistentPropertyMap.