Unabstracted IDGenerator; removed access to generator from Driver.

System::IDGenerator: merged with DummyIDGenerator implementation; removed abstraction.¹
System::Driver: made IDGenerator member non-ref, replaced IDGenerator ctor with default ctor.

1: There is a defect with the stdlib disallowing the ctor to be
noexcept-implicit.
Moved ownership of hives from Datastore to Driver.

IO::Datastore: removed all ownership of Hive::Unit.
System::Driver: added collection for hives, reworked placehold_hive().