Aura consensus for parachains (#371)

* Update polkadot

* Migrate all uses of MQC heads to merkle proofs

* Mass rename `relay_parent_storage_root`

* Restore parachain-system tests

* Update polkadot and libp2p swarm for testing

* Collapse match into an if let

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Start with something

* Update Substrate & Polkadot

* Start to make it compile

* Make it compile

* Begin with something

* Yep

* I'm a hacker

* Bring back the builder

* Make it work in some way

* Compile

* Parachains use their own "slot"

* Adds cumulus-pallet-aura

* Wrap AuRa import queue to disable equivocation checking by default

* Pass slot duration

* Check the seal when validating a block

* Adds missing file

* Try to make the seal working

* Fix it

* Some fixes

* Bring in the latest features to cleanup the code

* Update and make it compile

* Improve the import

* Start fixing

* More work

* Fix fix fix

* Make everything compile

* Small cleanups

* Rename and more docs

* Docs

* Fixes fixes fixes

* Update rococo-parachains/src/chain_spec.rs

* Update client/consensus/aura/src/lib.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Update client/consensus/aura/src/lib.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Update primitives/parachain-inherent/Cargo.toml

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Update primitives/parachain-inherent/Cargo.toml

* Update primitives/parachain-inherent/Cargo.toml

* Update primitives/parachain-inherent/Cargo.toml

Co-authored-by: Sergei Shulepov <sergei@parity.io>
Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>