Test layout with mixed font sizes.

Deduplicate pagination code.

Test Rich pagination.

Implement Rich pagination.

Qualify Plain pagination.

Test Rich line wrapping.

Test Rich layout.

Limit internal imports in interface tests.

Make paginateAll generic.

Implement piecewise paragraph construction.

Refactor pagination to allow rich layout.

Convert plain layout to a wrapper over rich layout.

Deprecate ParagraphConstruction.

Deprecate the Plain interface.

Add public Rich interface.

Add basic support for Rich layout.

Embed TextOptions in ResolvedSpan.

SpanOptions are relegated to the legacy Plain interface, and
TextOptions are now being used internally.

Define tree for structuring paragraph content.

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.