Add CudaSlice::split_at_mut and CudaViewMut::split_at_mut

[dkales]

This is an attempt to address #233.

Since this is code that touches some aspects of the lifetimes of CudaView and CudaViewMut, this could use some extra scrutiny to check if the safety invariants of the involved types are still ok.

I removed the root field of both CudaView and CudaViewMut, since it's only purpose was AFAIK, to keep track of the lifetimes of the respective structs. I have moved this into a PhantomData which should serve the same purpose.

Furthermore, I have now added the split_at_mut methods to CudaSlice and CudaViewMut, which mimic the behavior of split_at_mut of the standard library.

The old method of storing a &mut to the cuda device pointer does not work for splitting, since it would mean that there exist two mutable references to the same value (the u64 device ptr value) at the same time, which AFAIK is always unsound. Therefore the removal and moving the lifetimes into PhantomData as discussed above.

[coreylowman]

Since this is code that touches some aspects of the lifetimes of CudaView and CudaViewMut, this could use some extra scrutiny to check if the safety invariants of the involved types are still ok.

Yeah we could probably add some doctests that should fail to each of these methods. The main thing we have to make sure still works properly is the ownership/borrow semantics. We need to check if moving the references from the root variable to PhantomData changes these. Ideally would want CudaSlice/CudaViewMut to have the same behavior as a Vec<T>/&mut [T].

[dkales]

I have added some doc-tests to show the borrowing behavior of CudaView and CudaViewMut, and also rebased on main

[dkales]

As a sidenote the no-std feature is incompatible with the tests, since they use Vec.
I have added 3 more commits addressing this using the no-std-compat dependency already used.
Let me know if you want these or if they should be split into a separate PR.

[coreylowman]

It's fine to include here, will merge after piplines pass. Thanks for adding this feature!