`NewNEO()` and `NewGAS()` methods are trying to initialise
both `onPersist` and `incBalance` methods of NEO and GAS AFTER
nep5TokenNative is set to the VALUE of created nep5 token.
In this situation an attemmpt to call the corresponding native contracts
methods (e.g. transfer native GAS) leads to contract invocation failure,
as far as `nep5TokenNative.incBalance` method is nil.
Fixed this by initializing both `onPersist` and `incBalance` methods
before getting the value of nep5 contract.
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).
