Store span index in ResolvedSpan.

Link to Span using a generic wrapper.

Calculate fragment position continuously.

This merges the various "arrange" functions into the layout functions,
which will be used to detect overflows for line breaking.

Implement absolute line heights with half-leadings.

Calculate "normal" line height and pen position.

Use CSS-compatible line height settings.

Upgrade Fragment to contain pen position.

Use CSS3 terminology for fragments and containing block.

Avoid reimplementing Data.Text.Lazy.fromStrict.

Implement "plain" interface.

Use Rect in "plain" interface.

Create "plain" interface.