Fix class hierarchy diagram and docs for explicit nulls

[bishabosha]

http://dotty.epfl.ch/docs/reference/other-new-features/explicit-nulls.html

currently the link for the class hierarchy image is broken, and there are no links in the sidebar

[odersky]

Actually, the type hierarchy image still does not show.

[odersky]

In fact the diagram is outdated, and the surrounding text as well. Null is now a value type, not a subtype of Any. The docs and the diagram need to be fixed.

[odersky]

The link is good now, but the text and diagram need to be fixed.

[noti0na1]

@odersky Currently, Null is still a subtype of Any.

I tried to simply change the parent of NullClass to AnyValType, but this doesn't work. There are some errors in the Erasure phase.

Should we also add it to ScalaValueTypes?

[milessabin]

I'm a little concerned that code which matches against AnyVal scrutinees now has to be prepared to handle null. Granted, AnyVal isn't sealed, and yes, Null is no different from any other non-primitive subtype of AnyVal, which also need to be handled, but I can see this causing some surprises.

[sjrd]

@milessabin Honestly that sounds like a totally made up concern to me. I've never seen any code trying to exhaustively match on an AnyVal. And if there was, either it was already broken because it didn't handle custom value classes; or it was in fact correct because only some specific subtypes reached that match, but in that case there would be no new null reaching such code and hence it would stay correct.

[bishabosha]

This issue was about the diagram not showing, and maybe not being consistent with the implemented solution. However the diagram does show and it is consistent with the current state of things. Another issue should be opened for the status of where Null should go in the type hierarchy