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ARROW-13130: [C++] Add decimal support to arithmetic kernels

This adds decimal support for the following kernels (and _checked variants where applicable): abs, acos, add/sub/mul/div, asin, atan, atan2, ceil, cos, floor, hash_stddev, hash_tdigest, hash_variance, is_finite, is_inf, is_nan, ln, log1p, log2, logb, mode, negate, power, quantile, round, round_to_multiple, sign, sin, stddev/variance, tan, tdigest, trunc

Most kernels cast decimals to double and proceed. Some, including rounding, directly operate on decimals. Aggregate kernels directly operate on decimals (and cast to double inline) since DispatchBest is not usable for the aggregate nodes (at least, unless we also reimplement implicit casting there).

Additionally, ValidateFull for scalars/arrays now checks FitsInPrecision. A number of tests were adjusted to account for this.

Closes #11313 from lidavidm/arrow-13130

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>