Msg: removed unused module.
Cmd/Data: moved to Cmd/Object.¹

1: Metadata is really a property of Object, and the existing
commands can only operate on Object.
Cmd::Unit: added command_id() to boilerplate; doc update.
Cmd::Object: replaced error reports with notify_exception_current().
Cmd: added missing #undefs for HORD_SCOPE_CLASS.
Cmd::Unit: renamed message commit() to commit_error().
Cmd/Object: added Cmd::Object::SetSlug.
Cmd::Unit: replaced command_id() with COMMAND_ID constant.¹

1: This will make it possible to switch on command ID in context
notifications.
Cmd::UnitBase: added ok_action(); tidy.