This lifts a bunch of string conversions higher up the stack, but allows
us to make the servo code use utf-8 unconditionally, and seemed faster
in my benchmarking (see comment 0).
It should also make a bunch of attribute setters faster too (like
setting .cssText), now that we use UTF8String for them (we couldn't
because we couldn't specify different string types for the getter and
setters).
Differential Revision: https://phabricator.services.mozilla.com/D99590
