Parallel layout tree construction

Several parts of the "construction" phase of layout are expensive:

- [x] Inline layout construction (which does text segmentation and shaping)
- [ ] SVG construction (which currently serializes and then re-parses the SVG)
- [ ] Table construction (less so)

Each of these contexts are independent of each other, so we should consider deferring these expensive parts of tree construction until after the initial tree traversal and then processing them in parallel.