[css-values] Serialization of nested calc()

In a recent email, Tab mentioned that calc() should be nestable, and
should just be identical to parentheses.

I'd like to propose that, when serializing nested calc(), flattening
it to parentheses as well, instead of keeping the original redundant
function name.

It seems in both the spec and the impls, we already have some kind of
simplifying process, thus I think flattening nested calc() to
parentheses seems reasonable. It would likely also make implementation
simpler.

- Xidorn