~jaro/balkon

ref: 75e3b64a230f131fcde0b12604a187aef4ee2853 balkon/src d---------
Separate interface-independent constants.
Add user data to spans.
Use line numbers for page breaks.
Add line numbers to laid out fragments.
Future-proof records with options.
Fix compatibility with base < 4.15.
Improve terminology around fragments.
Fix documentation: missing Paginable.
Fix documentation: missing SpanOptions.
Fix documentation: document remaining names.
Fix documentation: export dependencies of exported names.
Fix documentation: incorrect reference.
Fix documentation: references to non-imported names.
Fix documentation: references to local names.
Separate internal plain Paragraph/ParagraphLayout.
Expose text slicing functions.
Decompose text slicing.
Integrate BiDi reordering.
Implement BiDi resolved level reordering.
Represent lines with non-empty lists.

We can do this because we removed lines with zero size.
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