Slightly decompose the Plain module.

Update documentation and derived instances.

Move non-public modules into Internal namespace.

Remove unnecessary default implementation.

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.

Define SeparableTextContainer as its own class.

Minimise working with Data.Text.Internal.

Implement run splitting.