Add folly::Expected, an alternative to exceptions for non-throwing APIs that can...

Add folly::Expected, an alternative to exceptions for non-throwing APIs that can fail.

Summary:
Expected is like an Optional with extra state for reporting //why// the Expected is empty. Something like it is currently under review for inclusion in the C++ standard [1], and Andrei Alexandrescu has spoken about it [2]. It corresponds to the Either Monad in Haskell, where it is used as a return type of an API that has more than one failure mode.

By adding folly::Expected, we get a way to implement non-throwing APIs with a consistent and composable interface.

[^1]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4015.pdf
[^2]: https://channel9.msdn.com/Shows/Going+Deep/C-and-Beyond-2012-Andrei-Alexandrescu-Systematic-Error-Handling-in-C

Reviewed By: mhx

Differential Revision: D3522501

fbshipit-source-id: 48b8ea2dfbd0769f26ec84d2d52fd41db75dc05a