BigInt tests fail on 32-bit platforms

[brson]

In std::bigint there are several disabled tests.

[gifnksm]

I think this issue can be closed by #4414

[pnkfelix]

I'm not sure we can close the issue quite yet. Though several of the tests were re-enabled by PR #4414, there remains (solely) the fn test_shr() which has ignore directives for {x86, arm, mips}.

(Alternatively we could open a fresh issue just dedicated to fixing that case, but I do not think that would accomplish anything.)

[emberian]

Visiting for triage; nothing to add except that it's extra::bigint now.