Add support for c++20 spaceship operator <=>

[jafar75]

Extend uint types with support for three-way comparison operator (<=>).

Closes #295.

[jafar75]

@chfast I've no idea why gcc-santizers failing in tests, unrelated to this PR

[chfast]

Please rebase your PR. The CI build issue should be resolved.

[chfast]

Thanks for this work.

The main motivation for this is to make usage like below compile and work as expected.

struct W
{
    intx::uint256 v;

    friend auto operator<=>(W, W) = default;
};

See https://godbolt.org/z/aq6TEjqqs for editable examples.

This doesn't work however. I believe the reason is that using int as the result type has some limiting effects. We probably should return std::strong_ordering. Maybe reading more of the C++ reference can clarify this.

At least, please add some unit tests like in these examples and try returning std::strong_ordering.

[jafar75]

@chfast, thanks for the review.
I switched to strong_ordering in my local and the test in godbolt got passed. will push the changes soon.

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[chfast]

Thanks a lot.