Blocks carry full attestations for candidates (#42)

* statement table yields fully-attested candidates

* attestation types in polkadot-primitives

* propose block with fully-attested candidates in consensus

* some signature-checking logic in the runtime

* fix runtime compilation

* ensure attestations are full and without duplicate when checking

* fix consensus-service compilation

* add some tests

* use bitvec from crates.io now that it's published

* sign statements based on primitive statement's encoding

* remove some serialize bounds

* Fix error message with duplicate availability attestations

Co-Authored-By: rphmeier <[email protected]>