~jaro/balkon

ref: 2462c039c364112d9015b036fe9217368d7bf2a5 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.