Compare `Vector<Parser::ComponentValue>` directly instead of
serializing them into strings first.
This is required for the upcoming changes where we would compare
previous and new sets of custom properties to figure out whether we need
to invalidate descendant elements. Without this change `equals()` would
show up being hot in profiles.
UnresolvedStyleValue::create() has one user where we know if there are
any arbitrary substitution functions in the list of CVs, and two users
where we don't know and just hope there aren't any. I'm about to add
another user that also doesn't know, and so it seems worth just making
UnresolvedStyleValue::create() do that work instead.
We keep the parameter, now Optional<>, so that we save some redundant
work in that one place where we do already know.
This is very hacky and wrong, but it means there's one place to fix,
instead of one for UnresolvedStyleValue, and one for invalid
MediaFeatureValues which I'm about to implement.
