This is a fairly major overhaul of the CSS parser to support
line numbers in error messages.
Basically, instead of passing "slices" of tokens around, we pass
indexes into the token arrays to all of our sub-parsers, which
allows them to have access to previous tokens in certain cases.
This is particularly important for errors where stuff is missing
(vs. being wrong).
In testing this out I found a few more places to catch errors.
Now, `tools/test-all` calls a new program called `tools/tests-tools`
that runs unit tests in `test_css_parser.py` and 'test_template_parser.py`.
This puts 100% line coverage on tools/lib/css_parser.py.
This puts about 50% line coverage on tools/lib/template_parser.py.
