decimal 64/32 for low pricision decimal

[周宇睿(闻拙)]

Hi

I noticed that currently Arrow has Decimal128 as the only decimal type with 128 
bit width. However, for decimal values with precision smaller than 18, it's 
kind of wastful to use 128 bit while 64 bit is more than enough. I am wondering 
is it possible for Arrow to implement types like Decimal64 or even Decimal32 
for small pricsion decimal value? 

Thanks
Yurui 

from Alimail macOS