Preserve inheritance

With contract code like `contract Car is GroundVehicle` and `contract Truck is GroundVehicle` I get types generated for `CarInstance` and `TruckInstance` and `GroundVehicleInstance` but when I have a function expecting a `GroundVehicleInstance` I can't pass in the `CarInstance` without a type error. This is because the types returned by a given function are incompatible: `Type 'import("path/to/Car").AllEvents' is not assignable to type 'import("path/to/GroundVehicle").AllEvents'.` Those incompatible types are the generic parameter to the Promise> type used in responses to functions.

This code would be much more useful if the inheritance could be preserved and used in that way.