Allow shortening reborrows

Generating a call to `as_mut()` let to more restrictive borrows than
what reborrowing usually gives us. Instead, we change the desugaring to
reborrow the pin internals directly which makes things more expressive.

Author: eholk

core/src/pin.rs

@@ -1408,7 +1408,6 @@ impl<Ptr: DerefMut> Pin<Ptr> {
     ///     }
     /// }
     /// ```
-    #[cfg_attr(not(bootstrap), lang = "pin_as_mut")]
     #[stable(feature = "pin", since = "1.33.0")]
     #[inline(always)]
     pub fn as_mut(&mut self) -> Pin<&mut Ptr::Target> {