[racket] typed racket and interfaces

Asumu Takikawa writes:

 > There is not (yet) and I don't know how we will support it if we do.
 > Right now the only option is to remove interfaces from your code and use
 > types instead to enforce that methods are implemented.
...

 > and so on. Also, I'm not sure interfaces add all that much value when
 > you have a structural type system like TR does.

Interfaces provide functions that dispatch on the type of one of their
arguments. I don't see how that can be done with types.

Konrad