Replace big.Int with uint256.Int

[roman-khimov]

There is a promising library that we could reuse for our integer types (both in VM and native contracts): https://github.com/holiman/uint256. We need to try it out, benchmark and compare with our current implementation.

[roman-khimov]

For the first iteration we need to try keeping public RPC interfaces with *big.Int types, but use uint256 in the VM, full compatibility is important here. Then benchmark it with VM tests we have (Bench*) and neo-bench (of course unit tests must work too). Then we'll see if it's worth it, if yes, we can gradually expand usage of this type in other places.

[ZhangTao1596]

For the first iteration we need to try keeping public RPC interfaces with *big.Int types, but use uint256 in the VM, full compatibility is important here.

You mean we use uint256 inside evm but big.Int in arguments of evm public methods?
What about util.Uint256?

[roman-khimov]

util.Uint256 is just a fancy way to represent "big" (compared to Uint160) hash. See neo-project/neo#938 also. For numbers that are real numbers (in that they're used for some arithmetic operations) we use *big.Int now mostly. uint256 library is supposed to be faster, but many of our interfaces rely on big.Int and we can't just throw it away, hence the requirement to keep these interfaces for now, exploring how uint256 can help us at the same time (there are some benchmarks in the vm code).