IO: added field PropInfo::object_type; moved PropStateStore to its own header.

IO::PropInfo: added field object_type and ctors.
IO::PropStateStore: moved to own header; updated test.

IO/Ops:
	catch on all exceptions in load_prop(), not just Error;
	simplified prop stream construction;
	matched param names from declarations;
	corrected @file and @brief in doc, updated param names.

Object::Unit: corrected IO includes.
Object/Ops: made IDPrinter ctor take const object; doc fixes.
System::Driver: added docs for datastore_hive_pair, normalized ctor init-list.
aux: added std::deque alias.
System::Context: switched stage buffers to deque.
IO::PropInfo: added assignment operators.
Cmd::Stage: implemented complex serialization, hid murk details.