split Data.NumInstances into two modules for functions and 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 .