Check stackitem jsonization

[roman-khimov]

neo-project/neo#1569 added direct stack item jsonization, we already have one defined in our code, but we need to check that it conforms to this format.

[roman-khimov]

And we need to apply it to app exec results (both exec result's stack and notification's state now use it). It also should be used for vm stack prints.

[roman-khimov]

RPC invocations results also use it now.

[fyrchik]

C# ToJson fails for recursive structures, which may or may not be what we want for execution results, especially for FAULT states. I assume recursive structures are a of no use anyway.