From the CSS token side, we already have these in FlyString form. From
the generated code side, we can easily return FlyStrings instead of
StringViews. So, let's do that, and save some work converting back and
forth.
Some dimensions would always serialize in a canonical unit, others never
did, and others we manually would do so in their StyleValue. This
commit moves all of that into the dimension types, which means for
example that Length can apply its special rounding.
Our local serialization test now produces the same output as other
browsers. :^)
