rvalueification of Future::unit(): 1/n

Summary:
This is part of "the great r-valuification of folly::Future":
* This is something we should do for safety in general.
* Several of folly::Future's methods are lvalue-qualified even though they act as though they are rvalue-qualified, that is, they provide a postcondition that says, in effect, callers should act as though the method invalidated its `this` object (regardless of whether that invalidation was actual or logical).
* This violates the C++ principle to "Express ideas directly in code" (see Core Guidelines), and generally makes it more confusing for callers as well as hiding the actual semantics from tools (linters, compilers, etc.).
* This dichotomy and confusion has manifested itself by some failures around D7840699 since lvalue-qualification hides that operation's move-out semantics - leads to some use of future operations that are really not correct, but are not obviously incorrect.
* The goal of rvalueification is to make sure methods that are logically rvalue-qualified are actually rvalue-qualified, which forces callsites to acknowledge that rvalueification, e.g., `std::move(f).unit()` instead of `f.unit()`. This syntactic change in the callsites forces callers to acknowledge the method's rvalue semantics.

Reviewed By: yfeldblum

Differential Revision: D9308405

fbshipit-source-id: 3ae0001deef1aef203454112757f8f64c47831fc

folly/futures/Future.h

@@ -1404,7 +1404,10 @@ class Future : private futures::detail::FutureBase<T> {
   /// - Calling code should act as if `valid() == false`,
   ///   i.e., as if `*this` was moved into RESULT.
   /// - `RESULT.valid() == true`
-  Future<Unit> unit() {
+  Future<Unit> unit() && {
+    return std::move(*this).then();
+  }
+  Future<Unit> unit() & {
     return std::move(*this).then();
   }