Multiple soundness issues in cgc

Affected versions of this crate have the following issues:

1. Ptr implements Send and Sync for all types, this can lead to data
   races by sending non-thread safe types across threads.

2. Ptr::get violates mutable alias rules by returning multiple mutable
   references to the same object.

3. Ptr::write uses non-atomic writes to the underlying pointer. This means
   that when used across threads it can lead to data races.