Param and TypeParam are now Trees (facepalm!)
develops the current annotation trend to its logical conclusion

Now that all trees have annots, we can significantly simplify handling
of annotations.

First, annotating a tree becomes really simple - we just get Tree.withAnnots
and get rid of all `val annots: List[Annots]` in former implementors of HasAnnots.

Second, since we now have withAnnots, we actually no longer need Tree.Annotated
and Type.Annotated, which is quite cool!