Update Arc and Rc to use NonNull::cast to cast the inner pointers

This avoids an `unsafe` block in each case.

Author: jsgf

Diff for: src/liballoc/arc.rs

@@ -1001,14 +1001,9 @@ impl Arc<Any + Send + Sync> {
         T: Any + Send + Sync + 'static,
     {
         if (*self).is::<T>() {
-            unsafe {
-                let raw: *const ArcInner<Any + Send + Sync> = self.ptr.as_ptr();
-                mem::forget(self);
-                Ok(Arc {
-                    ptr: NonNull::new_unchecked(raw as *const ArcInner<T> as *mut _),
-                    phantom: PhantomData,
-                })
-            }
+            let ptr = self.ptr.cast::<ArcInner<T>>();
+            mem::forget(self);
+            Ok(Arc { ptr, phantom: PhantomData })
         } else {
             Err(self)
         }

Diff for: src/liballoc/rc.rs

@@ -644,15 +644,9 @@ impl Rc<Any> {
     /// ```
     pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<Any>> {
         if (*self).is::<T>() {
-            // avoid the pointer arithmetic in from_raw
-            unsafe {
-                let raw: *const RcBox<Any> = self.ptr.as_ptr();
-                forget(self);
-                Ok(Rc {
-                    ptr: NonNull::new_unchecked(raw as *const RcBox<T> as *mut _),
-                    phantom: PhantomData,
-                })
-            }
+            let ptr = self.ptr.cast::<RcBox<T>>();
+            forget(self);
+            Ok(Rc { ptr, phantom: PhantomData })
         } else {
             Err(self)
         }