Iterator support in the RPC binding generator

[roman-khimov]

On one hand it's pretty simple, any InteropInterface{} return item can be treated as an iterator by default (other types are very rare), but on the other we have no idea what's the type of an individual iterator element. Even #2767 doesn't help a lot without additional annotations.

[roman-khimov]

Something like storage.Find(..., returnValue interface{}) with type derived from returnValue can help in absence of generics (#2376).

[roman-khimov]

Generics can fix it easily though.