* start graphing message flow between subsystems

This commit includes messages originating from the first three subsystems
(alphabetically).

* use standard arrowhead names

* add messages from bitfield distribution && use circo layout

* finish adding all subsystem message traffic to graph

* reduce line spam

* start work on graphing v1 types

* finish graphing candidate-related structs

* show hash relations, format

* show provenance of ValidityAttestation

* add the rest of the v1 polkadot primitives

* add type diagram from polkadot::parachain::primitives

* start work digramming the path to backing

* diagram message flow from collation generation -> candidate selection

* document flow through CandidateBacking, CandidateValidation

* graph data flow through StatementDistribution

* trace data flow through PoVDistribution to Backing

* finish documenting node side subsystem data flow