Implement new operator dispatch semantics.

Key points are:
1. `a + b` maps directly to `Add<A,B>`, where `A` and `B` are the types of `a` and `b`.
2. Indexing and slicing autoderefs consistently.
Update the guide examples and try not to leave user hanging as to what
this `&x` sigil is all about.
Remove incorrect doc annotation, mark experimental since we haven't discussed in an API meeting
Better debug printouts