Auto merge of #149218 - theemathas:pin-coerce-unsized, r=oli-obk

Make PinCoerceUnsized require Deref



Also, delete impls on non-Deref types.

Pin doesn't do anything useful for non-Deref types, so PinCoerceUnsized on such types makes no sense.

This is a breaking change, since stable code can observe the deleted `PinCoerceUnsized` impls by uselessly coercing between such types inside a `Pin`.

There is still some strange behavior, such as `Pin<&mut i32>` being able to coerce to `Pin<&dyn Send>`, but not being able to coerce to `Pin<&i32>`. However, I don't think it's possible to fix this.

Fixes https://github.com/rust-lang/rust/issues/145081