Add support for floor division (// and //=)

The implementation of getChildren() and getChildNodes() is intended to
be faster, because it avoids calling flatten() on every return value.
But it's not clear that it is a lot faster, because constructing a
tuple with just the right values ends up being slow.  (Too many
attribute lookups probably.)

The ast.txt file is much more complicated, with funny characters at
the ends of names (*, &, !) to indicate the types of each child node.

The astgen script is also much more complex, making me wonder if it's
still useful.