Make Expression and Statement separable

[elazarg]

This is a bigger change than previous PRs related to #1783, so yeah, much churn :( This is mostly due to the need to split visit_list() to deal separately with Expression and Statement, and similarly for .node(), .nodes() etc. I think it is unavoidable.

The good news:

1. Expression and Statement can be separated, although in this PR they are still aliases, to be safe. To test this change, make Expression and Statement subclasses of Node.
2. It is almost over. Basically what's left is checkstrformat.py and stats.py which should not be hard, and replacing the type of type_map to a be a callable (there should be a name for this type). After that, there will be almost no mention of Node in the code.

No casts are needed, but there are 7 assertions, in places where the calling code assumes to be true anyway (these also need the separation during tests, otherwise they are vacuous). I think these assertions will not be needed if and when there will be ExpressionVisitor and StatementVisitor.

I will not be available for the next two days. Happy new year everybody!

[gvanrossum]

Happy new year!