Store vertical offsets inside ProtoFragment.

Intended to make these offsets manipulable during layout.

Internally mark hard line breaks.

Simplify code since run direction is always known.

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.