* refactor beefy mmr

* use plain vector of bytes for leaf extra

* update comment

* update comments

* remove unused vars

* Use sp_std::vec::Vec

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* make extra data generic

* fix tests

* refactor beefy-mmr

* Update frame/beefy-mmr/src/lib.rs

* minor fix

* fmt

* Update frame/beefy-mmr/src/lib.rs

Co-authored-by: Adrian Catangiu <adrian@parity.io>