Merge CSS numeric factory functions into CSS typed OM

[foolip]

#2000 added numeric factory functions (CSS.px() and friends) as a separate feature, but to my eye this is just part of CSS Typed OM and not useful in its own right. The functions all return a CSSUnitValue instance, but is there anything one can do with those objects without using the reset of CSS Typed OM?

A merge should wait until after #91 so that the old ID can be redirected.

cc @andruud @ddbeck @jamesnw

[jamesnw]

This was always viewed as a junk drawer, with more context in #1847. The idea was that #971 would allow us to have composite features.

I don't think it's usable apart from Typed OM, but it was split into a separate feature since it does have a different support story. If #2820 collapses that support story, then I think it makes sense to merge.