eth-prover: incorrect behavior of Ethereum Merkle Patricia Trie proof verification

[abacabadabacaba]

During code review, I found the following issues with proof verification for Ethereum Merkle Patricia Trie (in contracts/near/eth-prover/src/lib.rs):

1. In function verify_trie_proof: the code that is supposed to split the bytes of the key into nibbles will not split a byte if actual_key.len() + 1 == proof.len(). This is incorrect.
2. In function _verify_trie_proof: the case node.len() < 32 is superfluous. The size of a receipt is always greater than 32 bytes, and therefore so is the size of any node in the trie.
3. In function _verify_trie_proof: the case key_index == key.len() is superfluous. The value may be only stored directly in a branch node if there are two keys such that one is a prefix of the other. In the trie of receipts, the keys are RLP encodings, and an RLP encoding can never be a prefix of a different RLP encoding.
4. In function _verify_trie_proof: the calls to Self::concat_nibbles are incorrect, because both key and key_end (or shared_nibbles) contain nibbles.
5. In function _verify_trie_proof: the condition expected_value.len() == 0 is incorrect. If the execution reached this point, the function should return false (however, since expected_value is an RLP encoding of a receipt, it cannot be empty).

[abacabadabacaba]

Fixed by #556.