Explicit return types for type class instances for Future/Task

Explicit return types for type class instances for Future/Task/Input …
Works around a bug in Scala 2.10.3-RC1, which is will be fixed
shortly in https://github.com/scala/scala/pull/2860.

But it's a good practice to annotate these types in any case.

Will close #376.

Review by @larsrh