Fix documentation of ProtoFragment after RTL placement.

Implement correct positioning of RTL runs.

Make SpanOptions a separate type.

Slightly decompose the Plain module.

Update documentation and derived instances.

Expose ParagraphConstruction.

Hide Rect union from public interface.

The function behaves in a specific way that should not be relied on by
other modules.

Use internal library to hide non-public modules.

Move non-public modules into Internal namespace.