We had `parse_calculated_value()` which parsed the contents of `calc()`,
and `parse_dynamic_value()` which parsed any math function, both of
which produce a CalculatedStyleValue, but return a plain StyleValue.
This was confusing, so let's combine them together, and return a
CalculatedStyleValue.
This also makes the other math functions work in
`StyleComputer::expand_unresolved_values()`.