Fix clippy::ptr_as_ptr (pedantic)

https://rust-lang.github.io/rust-clippy/master/index.html#ptr_as_ptr

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Author: andersk

gc/src/gc.rs

@@ -160,7 +160,7 @@ impl<
 
             // Create a pointer with the metadata of value and the
             // address and provenance of the GcBox.
-            let gcbox = set_data_ptr(value as *mut GcBox<T>, gcbox_addr);
+            let gcbox = set_data_ptr(value.cast::<GcBox<T>>(), gcbox_addr);
 
             // Move the data.
             ptr::addr_of_mut!((*gcbox).header).write(GcBoxHeader::new());