Add functions to interact with external memory

[ViliamVadocz]

• Add unsafe functions to crate::driver::result in a new external_memory module which wrap raw bindings for interacting with external memory.
• Add safe(?) wrapper in the form of two new types ExternalMemory and MappedBuffer which abstract away interactions with external memory and to manage cleanup on drop.

I did not add any tests because I don't yet know how to get a valid file descriptor using only CUDA. At least I confirmed that this interface works with a file descriptor exported from Vulkan (via vulkano crate). The code exists in my repo https://github.com/ViliamVadocz/nvidia-video-codec-sdk on the cudarc-external-memory branch (very experimental and unorganized).

[coreylowman]

This looks great to me. I'm inclined to merge this. Left a few questions.

Also this seems kind of like a mmap functionality in cuda? Am I interpreting this right? In llama-dfdx I've been experimenting with using mmap on CPU to not actually allocate on heap. Could this functionality be used to do something similar with cuda?

[coreylowman]

Nice addition, thanks for the contribution 👍