Allow projection in const functions

[jacobsa]

When I try to use a project_ref method in a const function I get an error:

error[E0015]: cannot call non-const method `union::_::<impl union::Running<F>>::project_ref` in constant functions
   --> storage/m5/async/task.rs:558:71
    |
558 |                 let future: Pin<&PinnedUnsafeCell<F>> = running_state.project_ref().future;
    |                                                                       ^^^^^^^^^^^^^
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

Is it possible to support this? It doesn't seem like there would be anything in the projection logic that can't itself be const.