~jaro/balkon

ref: 0c7129f8f8a5f337d97cd7ac62430cbd694ef9eb balkon/lib 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.