[css-typed-om] CSSNumericValue.to should take multiple type inputs

[shans]

... so you can "cast" a type to a calc expression - e.g. collapse a calc with inches, pixels and ems to a calc with just pixels and ems.

[tabatkins]

Now that we have toSum(), do we want to instead move this functionality there? So that toSum() (no args), just collapses as far as possible, but toSum("px", "em") returns something that has specifically "px" and "em" keys.

[shans]

This makes sense to me. I think this functionality is fairly specialist anyway, so it's OK for it to be slightly hidden away.

Note: we will need a constructor/factory function to convert sums back into values.

[tabatkins]

Done. toSum() now, appropriately, returns a CSSMathSum object, so there's no need for an additional factory function to turn the result back into a CSSNumericValue.