Memory pool support (cuMemPool*)

I noticed cudarc doesn't have any wrappers for the CUDA memory pool API (cuMemPoolCreate, cuMemAllocFromPoolAsync, etc.) beyond what's already in the sys bindings. From what I can know, cudarc uses cuMemAllocAsync internally for CudaSlice allocation, which goes through the default pool. But there doesn't seem to be a way to create custom pools, allocate from a specific pool, or do things like trimming unused memory. I came across this while looking at #514 and the downstream candle PR (huggingface/candle#3352) both deal with memory needing to persist across CUDA graph replays, and it seems like custom pools are how CUDA expects you to handle that. I was thinking this could be approached like this: 1. result-level wrappers in a new pub mod mem_pool for the core functions: create, destroy, trim_to, get/set attribute, and the device-level get_default_mem_pool/get_mem_pool/set_mem_pool. 2. A safe-level CudaMemPool type with Drop, plus something like CudaStream::alloc_from_pool() and CudaContext::default_mem_pool(). Though I'm not sure about a few things and would appreciate guidance: Should pool-allocated CudaSlices track which pool they came from? The pool struct (CUmemPoolProps) changed between 11.x and 12.x. So do we target a specific version? I am not too sure how this should interact with CudaGraph capture..? Happy to start with the result-level wrappers if this seems like a reasonable direction.