You can now call to_source() on Expansion, StringExpansion, Function
(and derived), Statement (and derived), Condition (and derived), and
StatementList to obtain make "source code" for that entity. This means
you can write out make files by constructing an appropriate set of
Statement instances.
This also implements __eq__ and __ne__ on all of the above. This is
being used in the tests to verify that the reformatting code works
properly (produces an equivalent StatementList).
khuey gave permission to land without review.
