Advanced state implementation

[dabasov]

1. Use sparse merkle tree as state storage instead of patricia merkle trie used now.
SMT described here https://eprint.iacr.org/2016/683.pdf has several benefits with regard to proof size and usage:

• use log(n)*hash_size bytes for proof
• easy leaf absence proof
• simple map based implementation

Implementation details:

• SMT is binary tree which is full and has predefined slots for all possible leafs, it means that it has 2^n leafs and 2^n(2^n - 1) nodes.
• Leafs are indexed sequentially with integers from 0 to 2^n - 1.
• Not leaf nodes contain hashes of it's siblings keccak256(append(left.value, right.value))
• Nodes are indexed with two integers which determine it's subtree: minor - leftmost leaf, major - leftmost leaf of its right sibling
• empty not leaf node has []byte{0} as it's hash when needed for hash calculation

2. Implement true snapshot state management.

• state tree repeats blockchain structure
• for each block only needed accounts are stored
• from newest to oldest block lookup for accounts is implemented