* convert border and padding to app units in flexbox
* convert margin to app units in flexbox
* cleanup, fmt
* add todo comment
* fmt
* add comment
* use Length instead of CSSPixelLength: they are same
This makes the names of flow relative geometry consistent with what is
used in the style crate and removes them from a module. With this change
it's more obvious what makes these types different from the ones in
`euclid`.
