Use ICU to find line break boundaries.

Implement line breaking on code point boundaries.

Implement run splitting.

Store span index in ResolvedSpan.

Link to Span using a generic wrapper.

Implement absolute line heights with half-leadings.

Use CSS-compatible line height settings.

Fix pretty printing for fragmented spans.

Upgrade Fragment to contain pen position.

Clean up ParagraphConstruction.

Use infix operators for constructing test data.

Implement "plain" interface.

Create "plain" interface.

Add module for working with rectangle coordinates.

Add explicit scale to test font.

Separate tests by module.