Integrate BiDi levels with spanToRuns algorithm.

Span/box boundaries no longer reset directionality.

BREAKING: Text with no strongly directional characters would
previously be treated as having no direction. It will now be
treated as having the direction defined in TextOptions.
This affects calculated positions.

Fix documentation: link to layoutRich.

Expose piecewise paragraph construction.

Consolidate tree types into one.

Implement piecewise paragraph construction.

Add basic support for Rich layout.