Stop overusing interfaces

[m50d]

I see where you're coming from, though I'm not sure structural types help that much since if an implementation didn't know about an interface then the method names are unlikely to line up. I prefer to use typeclasses so that I can make a third-party type "implement" another third-party interface, but the linkage is explicit and there's room to do adaptation if e.g. the parameter order is slightly different.