CSSUnitValue is a typed-om type which we will implement separately in
the future. However, it still seems useful to give our dimension values
a base class. (Maybe they could be templated in the future?) So instead
of deleting it entirely, rename it to DimensionStyleValue and make its
API match our style better.
A couple of arbitrary substitution functions require us to get or
produce some style value, and then substitute its ComponentValues into
the original ComponentValue list. So this commit gives CSSStyleValue a
tokenize() method that does so.
Apart from a couple of unusual cases like the guaranteed-invalid value,
style values can all be converted into ComponentValues by serializing
them as a string, and then parsing that as a list of component values.
That feels unnecessarily inefficient in most cases though, so I've
implemented faster overrides for a lot of the basic style value
classes, but left that serialize-and-reparse method as the fallback.
