Arkworks conversion example

[yshekel]

This PR:
(1) introduces a new Rust example for converting Arkworks types to ICICLE and back. It is meant to be used as reference for users using ICICLE.
(2) update to/from Montgomery trait and implementations to support HostOrDeviceSlice instead of DeviceSlice only so that it can work for host data as well.
(3) choose better c for cpu msm for way faster cpu (for non precompute case)

[krakhit]

Rust recommended to remove the double slicing here, since ark_scalars is already a slice. I think it allocates memory again when creating this extra slice. Minor change.

fn ark_to_icicle_scalars_async<T, I>(ark_scalars: &[T], stream: &IcicleStream) -> DeviceVec
where
T: PrimeField,
I: FieldImpl + MontgomeryConvertible,
{
// SAFETY: Reinterpreting Arkworks field elements as Icicle-specific scalars
let icicle_scalars = unsafe { &*(ark_scalars as *const _ as *const [I]) };

// Create a HostSlice from the mutable slice
let icicle_host_slice = HostSlice::from_slice(icicle_scalars);

let mut icicle_scalars = DeviceVec::<I>::device_malloc_async(ark_scalars.len(), stream).unwrap();
icicle_scalars
    .copy_from_host(icicle_host_slice)
    .unwrap();

// Convert from Montgomery representation using the Icicle type's conversion method
I::from_mont(&mut icicle_scalars, stream);
icicle_scalars

}

[yshekel]

updated but it doesn't seem to be faster