Add line wrapping tests with spans.

Make SpanOptions a separate type.

Move non-public modules into Internal namespace.

Add test for RTL text.

Test line splitting with many runs.

Add ignored prefixes and suffixes to test data.

This ensures that implementation does not rely on the texts having zero
offset inside their byte arrays.

Implement line breaking on code point boundaries.

Implement absolute line heights with half-leadings.

Use infix operators for constructing test data.

Implement "plain" interface.