Propagate BiDi levels to layout algorithm.

BREAKING: Text direction of the root box is now correctly used for
ordering fragments. Previously, the order was always LTR.

Move BiDi Level to separate module.

Integrate BiDi reordering.

Fix style: white space in exports and imports.

Decompose ProtoFragment construction.

Fix documentation of ProtoFragment after RTL placement.

Implement correct positioning of RTL runs.

Move non-public modules into Internal namespace.

Lay out Runs independently of Spans.