folly::copy

folly::copy

Summary:
[Folly] `folly::copy`.

Usable when you have a function with two overloads:

    class MyData;
    void something(MyData&&);
    void something(const MyData&);

Where the purpose is to make copies and moves explicit without having to spell out the full type names - in this case, for copies, to invoke copy constructors.

When the caller wants to pass a copy of an lvalue, the caller may:

    void foo() {
      MyData data;
      something(folly::copy(data)); // explicit copy
      something(std::move(data)); // explicit move
      something(data); // const& - neither move nor copy
    }

Reviewed By: markisaa, ericniebler

Differential Revision: D3462023

fbshipit-source-id: 6c777be288f2a7012c1b4b46dc988890b8662595