This should make it somewhat easier to experiment with alternative
representations in the future. To reduce churn, this commit leaves the String
field public, though.
Also, this will allow us to use the default String type to represent the IDL
USVString type, which explicitly forbids unpaired surrogates, ans as such is
a better match to the Rust String type.
The URL spec recently changed and the variour "mixins" interfaces are gone,
this commit updates our code and WPT accordingly.
The new expected failures related to HTMLAnchorElement and HTMLAreaElement's
attributes are due to their moving to the HTMLHyperLinkElementUtils interface,
which is not anymore in a separate <script class=untested> element.
