Add r < p check to FpMul

[SoraSuegami]

Description

This PR fixes a potential attack vector in the FpMul circom template such that a remainder r larger than or equal to a module p can pass constraints.
Example: a=4, b=4, p=5, q=2, r=6.
To fix this vulnerability, I added a range check r < p to the FpMul circom template.

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have discussed with the team prior to submitting this PR
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes

[saleel]

r_p_range_check can be renamed to r_p_lt_check (or similar) as we usually use "range check" for checking bit length range.

[saleel]

Looks good.
Added a minor comment.