Answers for Nested exposed by default #15

The README.md states

Nested

• Implement learnfp/nested/Nested.scala
  But all the code for functor and applicative instances is already implemented in the above file.

This issue is fairly straightforward, and the only necessary action is to replace the method definitions in Nested.scala with ??? to make them exercises.

PS: I am happy to submit a PR for this