`parse_as_foo()` implies that the Parser's internal data is used,
whereas `parse_a_foo()` implies that the passed-in data is used.
Also, made all the `parse_a_foo()` methods private, as they are only
required within the Parser, and this makes the API clearer to outsiders.
The `parse_a(s)_foo()` naming is a little awkward, but it comes from
section 5.3 of the spec, so seemed worth keeping:
https://www.w3.org/TR/css-syntax-3/#parser-entry-points