Add Computer.last_modified or similar

[khaeru]

With the planned deprecation of returning multiple Keys from Computer.add(), it should remain possible to inspect a larger batch of keys generated from a complex operation.

In order to do this, add a read-only property like Computer.last_modified that gives a collection (always) of the Keys (perhaps only 1) modified in the last operation. The contents would be overwritten for every new operation by calling a private method.