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

[byorgey]

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 diagrams/diagrams-lib#48 and
https://groups.google.com/forum/?fromgroups=#!topic/diagrams-discuss/c3KhwQO3L_g .

[conal]

I like this splitting. Thanks very much for the patch. I merged it, made a couple of very small tweaks, and released to hackage.

[byorgey]

Great, thanks!