Merkle proofs for receipts

[Kouprin]

The problem is to make sure that received receipts are trusty.

The common idea is to deliver a pair of (receipts, receipts_proof)
where receipts_proof is a Merkle proof of the receipts are received.

While chunk is producing, we calculate Merkle proof root and put it
into chunk header (check ClientActor::receipts_proof for more details).

Then we should check proofs in two cases:

1. Anyone who cares about shard and applying chunks completely
   must compare Merkle root with proof which is stored into chunk header.
2. Sending ChunkOnePart, we must calculate proofs for each subset of
   receipts related to some shard, as many as num_shards we have.
   Receiving ChunkOnePart, we must check that all proofs are given and
   all proofs guarantee that each subset contains in chunk receipts.

TEST PLAN

• sanity tests in chain and client folders
• cargo test --all-features --package near-client --test cross_shard_tx tests::test_cross_shard_tx -- --exact --nocapture
  (10 iterations)
• cargo test --all-features --package near --test run_nodes run_nodes_2 -- --exact --nocapture
• cargo test --package near --test stake_nodes test_validator_join -- --exact --nocapture

LOGIC REVIEWER

Alex Skidanov