Cmd::StageType: added Signal.
Added Cmd::Hive::Init command to replace IO::identify_all().
Data: added FieldMask enum and serialization functions.
Moved Rule::type_info::permitted_types to Node::Column;¹ removed System::Driver::register_rule_type().

Node::Column: replaced `type` property with `mask`.

Rule/Defs: removed type_info.

1: No reason to have to create a rule type for every field type
restriction desired, and if Rule::type_info only contains that,
there's no reason to have a special type_info for Rules.
Added Anchor object type and Anchor::UnitBasic;¹ doc tidy.

1: Needed something that doesn't carry any runtime fluff just for
building object hierarchies.
Changed object unit names.
doc/pages: added Cmd/Hive.dox and Object/Anchor.dox stubs.