The notion of NativeContractState shouldn't ever existed, native contract is a
contract and its state is saved as regular contract state which is critical
because we'll have MPT calculations over this state soon.
Initial minting should be done in Neo.Native.Deploy because it generates
notification that should have proper transaction context.
RegisterNative() shouldn't exist as a public method, native contracts are only
registered at block 0 and they can do it internally, no outside user should be
able to mess with it.
Move some structures from `native` package to `interop` also to avoid circular
references as interop.Context has to have a list of native contracts (exposing
them via Blockchainer is again too dangerous, it's too powerful tool).
