We make it explicit in the appropriate Block/Transaction structures, not via a
singleton as C# node does. I think this approach has a bit more potential and
allows better packages reuse for different purposes.
Two changes being done here, because they require a lot of updates to
tests. Now we're back into version 0 and we only have one type of
transaction.
It also removes GetType and GetScript interops, both are obsolete in Neo 3.
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).
