Add invalid selector tests from web-platfrom-test/selectors-api
Add valid selector tests from web-platfrom-test/selectors-api
Have :root match :firt-child and friends, and do some random refactoring.
Entirely skip selectors that are known to never match anything.
Reverse specificity order of match() results

For selectors of equal specificity we want insertion order, and rely on
.sort() being stable.
Fix specificity order.

(We also need to sort across buckets.)
Fix tests.
Simplify namespace test setup.
Fix case-sensitivity.
Fix :enabled, :disabled, and :checked
Fix :lang
Add from_xml_root() and from_html_root() instead of from_root()

… with a boolean argument.
This forces (hopefully) the user to make a concious choice.