depend on NumInstances 1.2 and import only Data.NumInstances.Tuple

Fixes #48.  Now we only export the Num (and Eq and Show) instances for
tuples.  Anyone wishing to use Diagrams.CubicSpline with a vector
space built over functions can import Data.NumInstances.Function themselves.

diagrams-lib.cabal

@@ -68,7 +68,7 @@ Library
                        diagrams-core >= 0.6 && < 0.7,
                        active >= 0.1 && < 0.2,
                        vector-space >= 0.7.7 && < 0.9,
-                       NumInstances >= 1.0 && < 1.1,
+                       NumInstances >= 1.2 && < 1.3,
                        colour >= 2.3.2 && < 2.4,
                        data-default >= 0.2 && < 0.6,
                        pretty >= 1.0.1.2 && < 1.2,

src/Diagrams/CubicSpline.hs

@@ -28,8 +28,8 @@ import           Diagrams.CubicSpline.Internal
 import           Diagrams.Path
 import           Diagrams.Segment
 
-
-import Data.NumInstances ()   -- for e.g. the Fractional (Double, Double) instance
+-- for e.g. the Fractional (Double, Double) instance
+import           Data.NumInstances.Tuple       ()
 
 import           Control.Newtype
 import           Data.Semigroup