Likely you don't need a generic implementation of Decimal.

What money do you want to represent, they all have a "atomic value".

For instance, when working with dollars store cents. When working with Eth, store wei. etc.

I can't think of a use case of money that needs decimals except maybe computing ownerships percentage. but that should never be stored, but rather computed.

Anything that needs to be converted is a "front end" view. all computations should use atomic store of value, thus no conversion is needed in contracts.