* Made new-style selectors public and restrict old-style to the crate.
* Improved output-formatting of new-style selectors.
* Partially implmented filtering out placeholders from new-style selectors.

The conversion from sass selectors to css selectors (i.e. the
evaluation of string interpolations in selectors) are still done
through the old css selectors.

The selectors context in a variable scope is now new selectors.

Writing new selectors is now don to `CssBuf` (handling different
formats) rather than through the `Display` trait.

Improved new pseudo selectors and some other selector handling using a
three-state option that can be "these selectors", "match-noting", or
"match anything".

* Evaluate sass selectors directly to new-style css selectors.
* Purge the old style css selectors from the code-base!

Remove remains of the old selector / logical selector divide.