Define BoxOptions and TextOptions for formatting.

Differentiating the two record types reflects the fact that the root
inline box cannot be styled but its text contents can.

Separate interface-independent layout algorithm.

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.