Cmd::IDFields: added IDFields + flag_initiator assign() variant; doc tidy.
Cmd::Status: moved 'waiting' after 'complete'; doc tidy.
Cmd::StageType: added Result.
Cmd/Ops: added printers for Cmd::IDField and Cmd::Stage.
System::Context: corrected host flag for output.
System::Context: added result deque and push_result().
System::Context: clarified purpose of push_output().
System::Context: reworked termination, made execution result a struct with named mvars.
Cmd: renamed HORD_CMD_STAGE_DATA_EMPTY() to HORD_CMD_STAGE_DATA_NO_SERIALIZATION().

Cmd: NodeCreate tidy.
Cmd: added Stage::get_data() and HORD_CMD_STAGE_DATA_OPEN_INHERIT().
Cmd: simplified stage definitions by calling HORD_CMD_STAGE_DEF_TYPE_INFO() in open macros.
Cmd::Stage: require standard layout for Data.
Cmd: added `self` variable for read/write stage data definition macros.
Cmd: added construct_stage() macros; tidy.
Cmd: added HORD_CMD_DEF_TYPE_INFO() macro to further reduce code duplication.