Make Decimal generic over Uint type

[uint]

Just to give you an idea of what this could look like if we wanted to be DRYer.

The idea is that eventually to implement Decimal<Uint256> we would only need to impl DecimalInnerType for Uint256.

The downside is I don't think Decimal::one or Decimal::zero can be be constant functions anymore because of this: rust-lang/rust#54469

It doesn't look like we can avoid a bunch of breaking changes here, so if we like this I'm thinking I'll wait for 1.0 and for now just create a Decimal256 that's pretty much a copy-paste of Decimal.

[webmaster128]

I'm sceptical this abstraction is really needed for the two cases we have. It creates new symbols that need to be documented and tought.

I'd favour a copy/paste solution for Decimal256 now. From there we can see what can be deduplicted.

[webmaster128]

Removing this from the 1.0.0 milestone. This is nice to have but should clearly not block the release. Decimal and Decimal256 should do everything we want right now.

[ueco-jb]

Does it make sense to keep it open?

[webmaster128]

Good to close this. I think macros are the better tool for what we need here to deduplicate some code.