Add extend instance for Array

While being defined for `List`, the `Array` primitive didn't previously
have a defined `Extend` instance. This has been added.

Author: Tom

src/Control/Extend.js

@@ -0,0 +1,9 @@
+"use strict";
+
+exports.arrayExtend = function(f) {
+  return function(xs) {
+    return xs.map(function (_, i, xs) {
+      return f(xs.slice(i));
+    });
+  };
+};

src/Control/Extend.purs

@@ -27,6 +27,11 @@ class Functor w <= Extend w where
 instance extendFn :: Semigroup w => Extend ((->) w) where
   extend f g w = f \w' -> g (w <> w')
 
+foreign import arrayExtend :: forall a b. (Array a -> b) -> Array a -> Array b
+
+instance extendArray :: Extend Array where
+  extend = arrayExtend
+
 infixr 1 extend as <<=
 
 -- | A version of `extend` with its arguments flipped.