Add MultiParamTypeClasses

Only one type is permitted as an instance head following changes in
Persistent. Add MultiParamTypeClasses to permit us to have more than one
type. This change is so trivial that it's better to do this than make
the Persistent dependency more strict.

Signed-off-by: Alexander Berntsen <alexander@plaimi.net>
Signed-off-by: Stian Ellingsen <stian@plaimi.net>
Reviewed-by: Stian Ellingsen <stian@plaimi.net>