Fix documentation: references to non-imported names.

Fix style: small Prelude imports.

Fix style: white space around operators.

Fix style: white space in exports and imports.

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.