~jaro/balkon

ref: 0e50058d5a6739068d79b08f6fac7fc29aa39f92 balkon/lib/Data/Text/ParagraphLayout d---------
Add user data to spans.
Add line numbers to laid out fragments.
Qualify Plain interface.
Future-proof records with options.
Fix style: white space in exports and imports.
Better format ParagraphConstruction documentation.
Remove (~) operator from ParagraphConstruction.

The tilde operator was causing compilation errors with older versions
of GHC, possibly due to a conflict with the lazy pattern match syntax.

It has been replaced by a pure Haskell alternative that uses function
application.
Make SpanOptions a separate type.
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.