Clarify spanLanguage and add basic ICU conversion.

Make SpanOptions a separate type.

Slightly decompose the Plain module.

Update documentation and derived instances.

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.

Implement "plain" interface.

Separate modules.