fix: use subtraction with reduce in AssertIsEqual

[ivokub]

Thanks @hussein-aitlahcen for reporting and helping to debug. It was a difficult one 👍

Description

To show that two field elements are equal, we instead show that the difference of the field elements is a multiple of emulated modulus. However, for computing the difference we used non-reducing version of subtraction to avoid infinite cycles. With the new mulmod implementation the reducing versions do not call AssertIsEqual anymore so the infinite cycles are averted. Previously, for some edge cases the difference may overflow scalar field and solving may fail (see here)

Fixes #1021

Type of change

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

Existing tests succeed.

• Added TestIssue1021 for the minimal reproducible example.

How has this been benchmarked?

Not benchmarked.

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[gbotrel]

One line PRs are easy to review :) .
This aside -- is it possible to write a test case that would have catch this in the emulated package? (non-regression purposes)

[ivokub]

One line PRs are easy to review :) . This aside -- is it possible to write a test case that would have catch this in the emulated package? (non-regression purposes)

Indeed! I forgot to include the regression test as I already had a test case when debugging for #1021, but didn't include here. I simplified it to make it as minimal as possible.