signed wrapper (#1283)

* 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 <[email protected]>

Co-authored-by: Robert Habermeier <[email protected]>