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

Following #2477 we now have Decimal128Type and Decimal256Type. I think it should be possible to make these impl ArrowPrimitiveType and consequently remove the need for DecimalArray entirely. Any logic for checked conversion (#2387) could be located and implemented in Decimal

This would provide a quick and easy way to increase the kernel support for decimals, as we would effectively get implementations "for free".

Describe the solution you'd like

I would like to treat decimals the same way we treat other constant width types

Describe alternatives you've considered

We could not do this, it was just an idea that occurred to me and may help to reduce the second-class citizen status that decimals currently have.

Additional context

Was noticed whilst working on #2635

FYI @liukun4515 @HaoYang670