* add signed wrapper, typedef SignedStatement

* typedef SignedAvailabilityBitfield

* implement Signed wrapper

This is strictly an addition as of this commit; nothing is yet
changed in existing behavior.

* inline getters, remove review comment

* move EncodeAs, Signed from node::primitives to primitives::parachain

* Refactor SignedAvailabilityBitfield to use Signed

* don't double-encode real payload

This isn't an ideal solution, because it depends on the
implementation details of how SCALE encodes tuples, but OTOH
that behavior seems unlikely to change anytime soon.

* fix build errors

* cause the runtime to build properly with the new changes

Not sure why cargo check didn't catch this earlier; oh well.

* fix runtime tests and separate SignedStatement from SignedFullStatement

* better explain why CompactStatement exists

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

* introduce polkadot-node-primitives

* guide: change statement distribution message types

* guide: remove variant from `CandidateSelectionMessage`

* add a few more message types

* add TODOs

* Almost all messages

* NewBackedCandidate notification

* Formatting

* Use AttestedCandidate as BackedCandidate

* Update node/primitives/src/lib.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Fix the tests

* Bring in types from #1242

* Adds network bridge messages

* More message types from doc

* use fn pointer type

* Fixes from the review

* Add missing Runtime subsystem message

* rename to CandidateValidationMessage and fix tests

Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>
Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* establish new `node` folder for overseer, messages, and subsystems

* extract message types from overseer crate

* remove doc links