Sparrow interfaces

[lucteo]

We need to implement some kind of "interfaces" to be able to decouple different part of the application.

An interface, similar to a Java interface, is a collection of function pointers. We can have an unknown implementor of these functions; the callers will just use the interface directly, without knowing who implements them

This needs to work nicely with concept. Each time we declare an interface, we should also generate an implicit concept for the interface. This way, we may use the interface in static contexts, where the implementor is known at compile-time, thus contributing to faster code.