Many changes.

Reformatting -- long lines, "[ ]" -> "[]", a few indentation nits.

Replace calls to Node function (which constructed ast nodes) with
calls to actual constructors imported from ast module.

Optimize com_node (most frequently used method) for the common case --
the appropriate method is found in _dispatch.

Fix com_augassign to use class object's rather than node names
(rendered invalid by recent changes to ast)

Remove expensive tests for sequence-ness in com_stmt and
com_append_stmt. These tests should never fail; if they do, something
is really broken and exception will be raised elsewhere.

Fix com_stmt and com_append_stmt to use isinstance rather than
testing's type slot of ast node (this slot disappeared with recent
changes to ast).