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ARROW-8480: [Rust] Use NonNull well aligned pointer as Unique reference

This reuses a well-aligned non-null pointer to work around the ZSTs. Currently, the issue that is referenced with this commit can't make it into our library since it is obliged to use unstable AllocRef API. We can figure out that issue when everything stabilizes.

@paddyhoran @nevi-me @andygrove

Closes #7734 from vertexclique/vcq/nonnull-buffer-ptr

Authored-by: Mahmut Bulut <vertexclique@gmail.com>
Signed-off-by: Wes McKinney <wesm@apache.org>