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. :^)
This reverts commit 76daba3069.
We're going to need separate types for the JS-exposed style values, so
it doesn't make sense for us to match their names with our internal
types.
