Update BEEFY+MMR integration.

[tomusdrw]

This PR changes the way we construct merkle trees for parachains and BEEFY validator set.

Note that the pallet code that previously was part of common/src/mmr.rs is now part of BEEFY repository (paritytech/grandpa-bridge-gadget#236)

[ordian]

@drahnr could please help fixing spellcheck https://gitlab.parity.io/parity/polkadot/-/jobs/1016684?

[drahnr]

@drahnr could please help fixing spellcheck https://gitlab.parity.io/parity/polkadot/-/jobs/1016684?

Working on a spellcheck fixing manifest, WIP.

For the particular CI failures:

error: spellcheck(Hunspell)
    --> /builds/parity/polkadot/runtime/rococo/src/lib.rs:858
     |
[2021-07-15T15:11:30Z WARN  cargo_spellcheck::suggestion] Unable to get terminal size. Using default: 80
 858 |  See [MmrLeafVersion] type documentation for more details.
     |       ^^^^^^^^^^^^^^
     |   Possible spelling mistake found.
error: spellcheck(Hunspell)
    --> /builds/parity/polkadot/runtime/rococo/src/lib.rs:858
     |
[2021-07-15T15:11:30Z INFO  cargo_spellcheck::action] ❌ /builds/parity/polkadot/runtime/rococo/src/lib.rs : 2
 858 |  See [MmrLeafVersion] type documentation for more details.
     |       ^^^^^^^^^^^^^^
     |   Possible spelling mistake found.

should become

[`MmrLeafVersion`]

@ordian which is why the pass currently is {"build_allow_failure":true} to avoid an hold-ups until the spellcheck-fixup manifest is complete

[tomusdrw]

CC @musnit @philipstanislaus

[philipstanislaus]

Nice, thanks 🙏

[adoerr]

LGTM. I did a smoke test with rococo-local.