Better documentation generation

[Ary Manzana]

One thing I'd really like to see in the documentation generated by ddoc is:
- for an interface: the superinterfaces[1], the known subinterfaces and 
the known classes implementing it
- for a class: the superclass[1] and the known subclasses

While browsing documentation, this is very useful for picking a 
particular implementation of an interface (i.e. which List should I use? 
ArrayList? LinkedList? etc.). Is it hard to implement?

Regards,
Ary

[1] While these are already shown, there is no link to go to their 
documentation.