Add abs_diff function for math package

[y-pakorn]

This should be a part of #1186

Add the abs_diff function for calculating the absolute difference in cosmwasm-math package as suggested

[webmaster128]

Nice, thank you. Can you make them const? Also I'd use other: Self consistently which seems a bit cleaner than the various sort of Into types.

[y-pakorn]

const abs_diff relies on primitive abs_diff which make the function not applicable to const identifier on Uint256 and Uint512 because the uint package has no native constant function to evaluate order and operation without panics.

We might just not use const abs_diff altogether (just normal abs_diff) and make all the functions able to run on Rust version lower than 1.60.0 too.

[webmaster128]

I see. Our < and - implementations are not const. Only the standard library can do const trait implementations.

[webmaster128]

Okay, let's make the Uint64/Uint128/Decimal case const. For the others we find a different implementation to constify them later.