1. `System.Contract.CallNative` expects version on stack.
2. Actual method is determined based on current
instruction pointer.
3. Native hashes don't longer depend on NEF checksum.
But don't change the way we process/store transactions and blocks. Effectively
it's just an interface for smart contracts that replaces old syscalls.
Transaction definition is moved temporarily to runtime package and Block
definition is removed (till we solve #1691 properly).
