Add S.fromEither to pull out values from either

S.fromEither :: b -> Either a b -> b

Closes: sanctuary-js#130

index.js

@@ -2060,6 +2060,24 @@
         return either.isLeft ? Nothing() : Just(either.value);
       });
 
+  //# fromEither :: b -> Either a b -> b
+  //.
+  //. Takes a default value and an Either, and returns the Right value
+  //. if the Either is a Right; the default value otherwise.
+  //.
+  //. ```javascript
+  //. > S.fromEither(0, S.Right(42))
+  //. 42
+  //.
+  //. > S.fromEither(0, S.Left(42))
+  //. 0
+  //. ```
+  S.fromEither =
+  def('fromEither',
+      {},
+      [b, $Either(a, b), b],
+      function(x, either) { return either.isRight ? either.value : x; });
+
   //. ### Alternative
 
   //  Alternative :: TypeClass