Remove I8/Int distinction.

The distinction was meant to provide additional type safety, but
inconsistency with other used libraries required too many workarounds
for this to be beneficial.

Minimise working with Data.Text.Internal.

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.