Make `gpu-poly` module more memory safe

[andrewmilson]

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 #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++.