`make_mp_identity` function?

In my code when I glue together boost.hana and boost.mp11 kind of MPLs I use my own `make_mp_identity` function defined as following:

```cpp
template
inline constexpr mp_identity make_mp_identity(T) noexcept { return {}; }
```

Would it be useful to add this function to mp11 library? Probably, the function should be named `mp_make_identity` rather that `make_mp_identity`.

I can make a pull request for it if needed.