Restructure the data store.

 - Facades: Transaction and Data Store
 - Internal: Transaction is a stack of operations, DataStore is a stack of transactions.
 - When client code commits or rolls back transaction, facade forwards to real transaction, which tells the internal data store to apply the operations
 - Move the logic for each action into the operation so that we fully utilise polymorphism

All tests pass. But I'm quite certain the tests are insufficient...

You'll also need to add virtual dtors to the following classes:
 - You::DataStore::Internal::IOperation
 - You::DataStore::Internal::PostOperation
 - You::DataStore::Internal::PutOperation
 - You::DataStore::Internal::EraseOperation
In C++, we can define the semantics of each operator using operator overloading, because
each operator is essentially just a method.

So `foo = goo` is equivalent to `foo.operator=(goo)`

By default, this is implemented by the compiler for us already, so we need to disable it in this case.=