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GH-35581: [C++] Store offsets in scalars (#36018)

ArraySpan contained scratch space inside itself for storing offsets when viewing a scalar as a length=1 array. This could lead to dangling pointers in copies of the ArraySpan since copies' pointers will always refer to the original's scratch space, which may have been destroyed.

This patch moves that scratch space into Scalar itself, which is more conformant to the contract of a view since the scratch space will be valid as long as the Scalar is alive, regardless of how ArraySpans are copied.
* Closes: #35581

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>