perf: mark the result of builder.IsZero as boolean to save constraints when used in future

[winderica]

Description

Hi there, thanks for your amazing work!

In the current implementation of gnark, builder.Select introduces a constraint to enforce that the condition is boolean. However, when the condition is from the result of builder.IsZero, this constraint is unnecessary, as builder.IsZero always returns a value m that is guaranteed to be either 0 or 1.

This PR marks m in builder.IsZero as boolean, so that we can save 1 constraint when using the result in subsequent operations like Select.

Type of change

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

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas (N/A)
• I have made corresponding changes to the documentation (N/A)
• I have added tests that prove my fix is effective or that my feature works (N/A)
• 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 (N/A)

[CLAassistant]

All committers have signed the CLA.

[ivokub]

Hi @winderica - thanks for the contribution. I have been busy before holidays to review but will definitely get to it next year. Sorry for the delay.

I think the fix would be useful but I would first have to see thoroughly the test suite to see if there are not unintended side-effects.

[gbotrel]

lgtm 👍