pallet-mmr: move offchain logic to client-side gadget (#12753)

* Move MMR utils methods from pallet to primitives

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

* Add method to MmrApi

* Move forks expanding logic from babe to primitives

* Implement MMR gadget

* Remove prunning logic from the MMR pallet

* Code review changes: 1st iteration

* Replace MaybeCanonEngine with CanonEngineBuilder

* fix mmr_leaves_count() for kitchen sink demo

* Update client/merkle-mountain-range/src/canon_engine.rs

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

* Code review changes: 2nd iteration

* fix INDEXING_PREFIX

* impl review comments

* add documentation and minor rename

Signed-off-by: Serban Iorga <serban@parity.io>
Co-authored-by: Adrian Catangiu <adrian@parity.io>

substrate/bin/node/runtime/src/lib.rs

@@ -2066,6 +2066,10 @@ impl_runtime_apis! {
 			Ok(Mmr::mmr_root())
 		}
 
+		fn mmr_leaf_count() -> Result<mmr::LeafIndex, mmr::Error> {
+			Ok(Mmr::mmr_leaves())
+		}
+
 		fn generate_proof(
 			block_numbers: Vec<BlockNumber>,
 			best_known_block_number: Option<BlockNumber>,