arrow

GH-15072: [C++] Move the round functionality into a separate module (#15073)

Splits the scalar_arithmetic.cc module into two parts.  This should make it easier for upcoming changes to round functionality to be reviewed easier.  A side effect of this refactoring is that it illuminated the lack of benchmarks for round functionality which will be addressed in a future PR.

* Closes: #15072

Authored-by: David Sisson <EpsilonPrime@users.noreply.github.com>
Signed-off-by: Yibo Cai <yibo.cai@arm.com>