Mark Node types as Statements or Expressions

[rwbarton]

This doesn't actually do anything yet, as Statement and Expression are
just synonyms for Node. But it's part of the work that will need to be
done for #1783, and for now it serves as (unchecked) documentation.

[rwbarton]

I got about this far before determining that now is not really the best time to go ahead with this refactoring (#1783). Since this was a non-trivial amount of work to do, I put it in this form so that it could be merged now before doing the actual refactoring.

Locally I had some other (very rough) changes that tested these annotations to some extent, but mostly they should be considered untested for accuracy.

[rwbarton]

There are also some rough edges here; for example I marked Var as a Statement because it has an accept method and it isn't an Expression (the type checker doesn't give it a type), but of course it isn't really a statement either. Perhaps it needs its own category.

[gvanrossum]

Merging now as I have a branch where I'm adding more nodes and I might as well rebase before I get too far along. Thanks for this first pass!