I decided against the name flexRow/flexCol as the "flex" prefix
is too general for these implementations, which are mostly just
useful for filling the space with the children. Flex box has a
lot more features than that, such as centering, wrapping,
justifying, etc., but I don't currently know how to design an
API that presents the full power of flex box without also
presenting the full complexity of it as well.
This commit also includes some tweaks to the impl of flexfill
to fix the behavior of children with 100% size along the main
axis, and also introduces support for NA flex values, which
sizes the flex item according to its contents.