This extracts the data buffer from a tensor without making it contiguous.

This will be useful for a tensor pool/arena in the rten crate.

This initializes a `Tensor<MaybeUninit<T>>` by copying data from an existing
view/tensor.

Improve buffer re-use during graph execution by adding a pool from which
operators can allocate output buffers, and into which buffers are added
when their ref count drops to zero (ie. when they are no longer needed
by subsequent graph execution steps). This significantly reduces how
often execution needs to allocate "fresh" buffers from the system
allocator and free them back.

In this initial implementation, a reference to the pool is passed to all
operators via `Operator::run`, but only a subset actually use the pool.
This subset was chosen to benefit the YOLOv8 example.

 - Add `pool` argument to `Operator::run`, specifying a pool from which
   operators should allocate their outputs

 - Create a pool at the start of graph execution and release it at the end.
   Intermediate values that are no longer needed are added to the pool after
   each operator runs.

 - Report the number of allocations from the pools and the hit rate (how often
   the pool was able to satisfy allocations) as part of timing info.

 - Modify an initial subset of allocators to allocate from the pool, based on
   what helps the YOLOv8 example.

If the `RTEN_USE_POOL` env var is set, the pool will be used. Otherwise the pool
is still created, but buffers are never added to it, so all allocations go
through the system allocator as before.