I didn't want to add another set of boilerplatey tree-walking methods,
so here's a general-purpose one. :^)
`for_each_effective_rule()` walks the tree of effective style rules, and
runs the callback on each one, in either pre- or postorder. The
previous `for_each_effective_style/keyframes_rule()` methods of
`CSSStyleSheet` are then reimplemented in terms of
`for_each_effective_rule()`, and we can get rid of their equivalents
elsewhere.
