Pass axis information when measuring a child's size

[nicoburns]

Objective

This enables Taffy to be integrated with Xilem.

See: linebender/xilem#140

Changes made

• Add RequestedAxis enum
• Add axis: RequestedAxis parameter to child measurement and plumb through to various places

Feedback wanted

• API design review
• Should we split apart the compute_child_layout methods on the PartialLayoutTree trait? Because the axis parameter is only really relevant to the "measure child" case rather than "perform layout" case. Also, the "measure child" case could return f32 instead of LayoutOutput in that case. Which would also reduce cache size.

[nicoburns]

Seeing as this is approved, I think I'm just going to merge this now (and we can get Xilem running off Taffy main), and follow up with further changes in separate PRs, which will likely include:

• Splitting the compute_child_layout method into 3 separate methods (eliminating the RunMode parameter).
• Replacing the get_layout_mut method with a set_layout method (which allows integrators of Taffy to more easily route layout data to a storage location of their choice)

[alice-i-cecile]

Yep, agreed with that. I didn't have strong opinions about the architecture here.