Add user data to spans.

Separate internal plain Paragraph/ParagraphLayout.

Expose text slicing functions.

Decompose text slicing.

Move paragraphSpanBounds to the correct module.

Fix style: white space in exports and imports.

Separate module for the ParagraphLayout type.

Add internal functions for "shaped runs" output.

Clarify meaning of Array in Paragraph.

Slightly decompose the Plain module.