Consolidate and deduplicate MMR API methods (#12530)

* histor. batch proof: make best block arg optional

* correct testing range

* make generate_batch_proof stub for historical

* merge generate_{historical_}batch_proof functions

* merge generate_{batch_}proof functions

* merge verify_{batch_}proof functions

* merge verify_{batch_}proof_stateless functions

* remove {Leaf}Proof

Not utilized by API anymore, so superfluous.
Removal consistent with prior changes to just use "batch" proof API.

* rename BatchProof->Proof

no need to qualify if only one universal proof type.

* cleanup

* expose verify_proof rpc api

* document verify_proof

* expose verify_proof_stateless rpc api

* add optional BlockHash to mmr_root rpc api

* fixup! expose verify_proof rpc api

* fix documentation phrasing

Co-authored-by: Adrian Catangiu <[email protected]>

* documentation grammar

Co-authored-by: Adrian Catangiu <[email protected]>

* define mmr error msgs together with error enum

Co-authored-by: Serban Iorga <[email protected]>

* fixup! define mmr error msgs together with error enum

* map decoding errors to CallError::InvalidParams

Co-authored-by: Serban Iorga <[email protected]>

* fixup! map decoding errors to CallError::InvalidParams

Co-authored-by: Adrian Catangiu <[email protected]>
Co-authored-by: parity-processbot <>
Co-authored-by: Serban Iorga <[email protected]>