Summary:
`SharedFunction` implements a pattern of a shared callable object that contains the same executable inside. It's similar to `std::function` with one important difference: when the object is copied, the stored function (and captured values) are shared between instances (not copied).
`SharedFunction` can be stored inside `std::function` because it's callable.
It useful in some scenarios, such as:
* When captured by `std::function` arguments are not copyable;
* When we need to replace the content of the callable later on the go.
We will use it in the coming diffs.
Reviewed By: mdvacca
Differential Revision: D14072078
fbshipit-source-id: 9df4ad2d1b92394e2dfef5c283f35c7c0bd4b500
