Make taffy::compute::compute_layout public

[nicoburns]

The combination of #238 and #246 means that it is now almost possible to use taffy without using the Taffy struct at all. One can provide their own storage type implementing LayoutTree with it's own methods for constructing a tree. In theory, one should then be able to call taffy::compute::compute_layout directly on this type. Unfortunately this function isn't public which is currently preventing this use case.

I suggest that we make this method public (perhaps it would be best to re-export as a top-level function?). We may also want to document the existence of the LayoutTree trait.

[alice-i-cecile]

Yep, I'm on board!