use folly::Function<void()> in folly::Executor interface

Summary:
This diff changes the definition of `folly::Func` from `std::function<void()>`
over to `folly::Function<void()>`. This mostly affects the interface of
`folly::Executor` and derived and related classes. By using
`folly::Function<void()>`, we allow to have lambdas capturing move-only types
passed to executors. This continues an effort to get rid of the potentially
dangerous `folly::MoveWrapper` by allowing to capture move-only types in lambdas
when using `folly::Future`, `folly::EventBase` and now `folly::Executor`.

Reviewed By: yfeldblum

Differential Revision: D3706449

fbshipit-source-id: 11c2cee32cb9f0298c39b7b1695a477777eeb3aa

folly/Executor.h

@@ -21,9 +21,11 @@
 #include <functional>
 #include <stdexcept>
 
+#include <folly/Function.h>
+
 namespace folly {
 
-typedef std::function<void()> Func;
+using Func = Function<void()>;
 
 /// An Executor accepts units of work with add(), which should be
 /// threadsafe.