Define and implement the Incrementable trait for uints

[Moliholy]

Currently the Incrementable trait is defined in frame-support and only used in asset-conversion and nfts. This prevents types like U256 to be used in such pallets, which is particularly inconvenient when facing Ethereum compatibility.

This PR redefines the Incrementable trait and implements it for uints, with the intention remove it from frame-support and use this one instead.

[bkchr]

That will not work. You basically just create a different trait here. To achieve what you want to achieve, you need to implement the trait for these types in the crate where the trait is declared. It is also done for the other fundamental types there: https://github.com/paritytech/polkadot-sdk/blob/master/substrate/frame/support/src/traits/storage.rs#L282