Simplify public module hierarchy

[nicoburns]

What problem does this solve or what need does it fill?

When a user browses the docs for Taffy, it can be confusing to know where to look as there are a lot of modules, some of which are much more important than others

What solution would you like?

• Make math module private: it has no public items anyway.
• Make the error module private and re-export TaffyError/TaffyResult as top-level types instead.
• Merge axis module into geometry
• Merge tree, node, data, layout and cache modules into the tree module:
  • LayoutTree, RunMode, SizingMode,SizeAndBaselines and Layout should live in the top-level tree module
  • NodeId, NodeData, and MeasureFunction should live in tree::node
  • cache should remain it's own module, just nested under tree
  • Taffy should live it's own taffy_tree module
  • All public types in tree should rexported from the top-level tree module
• Taffy should be renamed to TaffyTree

What alternative(s) have you considered?

Don't do this.

Additional context

[Weibye]

I'm in favour of the proposed structure. :+1|