Creating a flatten() method to get the inner Future<T> value out of a Future<Future...

Creating a flatten() method to get the inner Future<T> value out of a Future<Future<T>>.

Summary: Method will only be applicable for types of the form Future<Future<T>>, calling flatten() on a Future<T> where <T> != Future<R> for any <R> will result with a compile time error.

Test Plan: Added tests in FutureTest.cpp.

Reviewed By: hans@fb.com

Subscribers: ldbrandy, trunkagent, folly-diffs@, jsedgwick, yfeldblum, icanadi

FB internal diff: D1843581

Tasks: 6166903

Signature: t1:1843581:1424399517:7146924db3c238b77dd309f1d5916e44a7e81968