Improve parsing of static values

This PR is an evolution of https://github.com/sass/libsass/pull/679 which makes the static value look ahead more faithful to the Ruby sass version.

As a bonus this makes the follow tests pass:
- https://github.com/sass/sass-spec/blob/master/spec/libsass-todo-tests/15_arithmetic_and_lists
- https://github.com/sass/sass-spec/blob/master/spec/libsass-todo-tests/at-stuff

However upon further investigation the second spec is out of date and incorrect in 3.3+.