Interfaces documentation

Hi,

I noticed that interface fields are duplicated on the class implementation, see for instance interface here:
https://heaps.io/api/hxd/fs/FileSystem.html
and implementation there:
https://heaps.io/api/hxd/fs/BytesFileSystem.html

I think we should only list the methods/vars that have different signature - the way we do already with subclass overrides - as the implementation is most likely shared.