fix(integer): StaticSignedBigInt right shift

[tmontaigu]

Check-list:

• Tests for the changes have been added (for bug fixes / features)

[github-actions]

@slab-ci cpu_fast_test

[IceTDrinker]

Can you quickly indicate what wrong value we were getting before ?

[github-actions]

Pull Request has been approved 🎉
Launching full test suite...
@slab-ci cpu_test
@slab-ci cpu_integer_test
@slab-ci cpu_multi_bit_test
@slab-ci cpu_wasm_test
@slab-ci csprng_randomness_testing

[tmontaigu]

For i256::MIN >> 1 we were getting [0, 0, 0, u64::MAX], so 64 1 in the MSB then zeros
when we expect 2 1 in MSB and the rest to be zero

[IceTDrinker]

ok arithmetic shift

[tmontaigu]

Yes but we were putting the value of the sign bit all over the most significant block and instead of only in places where sign bits would actually need to be used as padding