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