Move erasure root out of candidate commitments and into descriptor (#2010)

* guide: move erasure-root to candidate descriptor

* primitives: move erasure root to descriptor

* guide: unify candidate commitments and validation outputs

* primitives: unify validation outputs and candidate commitments

* parachains-runtime: fix fallout

* runtimes: fix fallout

* collation generation: fix fallout

* fix stray reference in primitives

* fix fallout in node-primitives

* fix remaining fallout in collation generation

* fix fallout in candidate validation

* fix fallout in runtime API subsystem

* fix fallout in subsystem messages

* fix fallout in candidate backing

* fix fallout in availability distribution

* don't clone

* clone

Co-authored-by: Sergei Shulepov <sergei@parity.io>