pallet-mmr: generate historical proofs (#12324)

* BEEFY: generate historical proofs

Signed-off-by: Serban Iorga <serban@parity.io>

* Update frame/merkle-mountain-range/rpc/src/lib.rs

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

* Update primitives/merkle-mountain-range/src/lib.rs

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

* Update frame/merkle-mountain-range/src/lib.rs

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

* cargo fmt

* fix off-by-one in leaves powerset generation

* test all possible mmr sizes for historical proofs

* remove now redundant simple_historical_proof

* cargo fmt

Signed-off-by: Serban Iorga <serban@parity.io>
Co-authored-by: Adrian Catangiu <adrian@parity.io>
Co-authored-by: Robert Hambrock <roberthambrock@gmail.com>