Slightly decompose the Plain module.

Move non-public modules into Internal namespace.

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 run splitting.

Store span index in ResolvedSpan.

Implement absolute line heights with half-leadings.

Implement "plain" interface.