~jaro/balkon

ref: 520267372b6484d24cb7029e61dd885e07909db5 balkon/lib/Data d---------
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.