Document structural subtypes hierarchy

[tristanlatr]

typing.Protocol are the new way to declare interfaces. But it's much harder to process than zope interfaces because implementers do not require to explicitly declare implementing a specific protocol. So we'll have to do some matching.. The problem with this is the fact that we won't ever have a complete understanding of the type system, so we'll have to find middle ground somewhere.

@runtime_checkable decorator make things a little bit simpler since it skip the type checks to only matches the names of the members, so that's a good starting point.