Convert plain layout to a wrapper over rich layout.

Embed TextOptions in ResolvedSpan.

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

Separate interface-independent layout algorithm.

Separate interface-independent constants.

Add user data to spans.

Add line numbers to laid out fragments.

Fix compatibility with base < 4.15.

Fix documentation: references to local names.

Separate internal plain Paragraph/ParagraphLayout.

Decompose text slicing.

Integrate BiDi reordering.

Represent lines with non-empty lists.

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

Remove lines with zero size.

These lines would otherwise affect line numbering.

Do not re-export through the Plain module.

Clarify comment about BiDi.

Fix style: white space in lambdas.

Fix style: white space around operators.

Fix style: white space in exports and imports.

Mark some lists as explicitly non-empty.

Separate module for the ParagraphLayout type.