BEEFY: Simplify hashing for pallet-beefy-mmr (#12393)

* beefy-mmr: reuse sp_runtime::traits::Keccak256

* beefy-mmr: use sp_runtime::traits:Hash for generating merkle proofs

* beefy-mmr: use sp_runtime::traits:Hash for validating merkle proofs

* beefy-mmr: remove primitives::Hasher and primitives::Hash

* fixes

* beefy-mmr: reduce the number of generic parameters for merkle_root()

* fix

* compute upper Vec capacity more accurately