Remove pin::Unpin reexport and add Unpin to the prelude.
Change Pin associated functions to methods.
Rename get_mut_unchecked_ to get_unchecked_mut
Remove impl Unpin for Pin
Mark Pin repr(transparent)
This lets you write methods using `self: Rc<Self>`, `self: Arc<Self>`, `self: Pin<&mut Self>`, `self: Pin<Box<Self>`, and other combinations involving `Pin` and another stdlib receiver type, without needing the `arbitrary_self_types`. Other user-created receiver types can be used, but they still require the feature flag to use.
This is implemented by introducing a new trait, `Receiver`, which the method receiver's type must implement if the `arbitrary_self_types` feature is not enabled. To keep composed receiver types such as `&Arc<Self>` unstable, the receiver type is also required to implement `Deref<Target=Self>` when the feature flag is not enabled.
This lets you use `self: Rc<Self>` and `self: Arc<Self>` in stable Rust, which was not allowed previously. It was agreed that they would be stabilized in #55786. `self: Pin<&Self>` and other pinned receiver types do not require the `arbitrary_self_types` feature, but they cannot be used on stable because `Pin` still requires the `pin` feature.
Rename `CoerceSized` to `DispatchFromDyn`, and reverse the direction so that, for example, you write
```
impl<T: Unsize<U>, U> DispatchFromDyn<*const U> for *const T {}
```
instead of
```
impl<T: Unsize<U>, U> DispatchFromDyn<*const T> for *const U {}
```
this way the trait is really just a subset of `CoerceUnsized`.
The checks in object_safety.rs are updated for the new trait, and some documentation and method names in there are updated for the new trait name — e.g. `receiver_is_coercible` is now called `receiver_is_dispatchable`. Since the trait now works in the opposite direction, some code had to updated here for that too.
I did not update the error messages for invalid `CoerceSized` (now `DispatchFromDyn`) implementations, except to find/replace `CoerceSized` with `DispatchFromDyn`. Will ask for suggestions in the PR thread.
