Split Data.NumInstances into two modules, one for function instances and one for tuples

This way, someone wishing to get (say) just the tuple instances can
import `Data.NumInstances.Tuple` and not get the function instances.
This change should be completely backwards-compatible.

See https://github.com/diagrams/diagrams-lib/issues/48 and
https://groups.google.com/forum/?fromgroups=#!topic/diagrams-discuss/c3KhwQO3L_g .