Use ArrayAccessor in Decimal128Iter and Decimal256Iter

[viirya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

We have DecimalIter which iterates i128 values in Decimal128Array. One issue is that we should use Decimal128 which is formal value in Decimal128Array now. i128 is kept for back-compatibility purpose only. We should also follow other arrays to have iterators that leverage ArrayAccessor.

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