Calc simplification (which I'm working towards) involves repeatedly
deriving a new calculation tree from an existing one, and in many
cases, either the whole result or a portion of it will be identical to
that of the original. Using RefPtr lets us avoid making unnecessary
copies. As a bonus it will also make it easier to return either `this`
or a new node.
In future we could also cache commonly-used nodes, similar to how we do
so for 1px and 0px LengthStyleValues and various keywords.
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.
