Replace the `*_impl` proxy creation functions with type traits that name the
selected lifetime model, so that the models can be reused by types other than
`proxy` (e.g. an upcoming `box`). No observable behavior change.

- Rename the internal `allocated_ptr` class template to `wide_ptr`, and add
  `allocated_ptr`, `owned_ptr` and `shared_ptr` type traits that select a
  lifetime model instead of constructing a `proxy` directly. The creation
  functions now construct `proxy<F>` from the selected pointer type.
- Let `inplace_ptr` ignore its first constructor argument so that all owning
  lifetime models share a uniform `(alloc, args...)` construction signature.
- Order the `T&& value` overload first in each creation function family, and
  renumber the corresponding specs to match.