Initial implementation of `FnPtr` trait

This commit is an initial implementation of the `FnPtr` trait as
described in the `fn_static` tracking issue, which consists of moving
the internally unstable `core::marker::FnPtr` to `core::ops::FnPtr`, as
well as changing the API. Because `NonNull` is used in the new `as_ptr`
signature, it was also turned into a proper lang item.