Make `gpu-poly` module more memory safe

Currently miniSTARK passes around *mut pointers when sharing memory addresses with GPU code in a rather unsafe manner. Would be great if this memory was wrapped in some way to allow rust to enforce memory safety. Will be good to keep the memory challenges with https://github.com/andrewmilson/ministark/issues/2 in mind with this work. A couple of the problematic functions to understand the issue:

• buffer_no_copy
• buffer_mut_no_copy

In addition to these changes any types being sent for processing to the gpu should be represented in memory as you would expect from C or C++.