Cuda: Consider using `cuMemAllocHost` for allocating bigarrays when the backend is Cuda.

[lukstafi]

 * Allocating
 * excessive amounts of memory with ::cuMemAllocHost() may degrade system
 * performance, since it reduces the amount of memory available to the system
 * for paging. As a result, this function is best used sparingly to allocate
 * staging areas for data exchange between host and device.