[Feature Proposal] Add child-w-* utilities to help declutter when all children are the same width

[robmcfadden]

I like having the option of specifying widths for direct children of parents on the child or on the parent when the children are all the same width. This helps to declutter the code a bit when you have, say, a flex container with dozens of children. You could have one class on the parent to rule them all and save yourself dozens of redundant (multiplied if you're changing widths responsively) classes in the HTML source.

child-w-* should have the same scale as the width utility, but instead of applying the width to itself, it should be applied to all of its direct children: > *

Happy to contribute a pull request if this sounds like a good idea!