FpVar::{is_eq, is_neq} only need two constraints

[winderica]

Description

Hi there, thanks for your amazing work!

This PR reduces 1 constraint in FpVar::{is_eq, is_neq} (and in turn, is_zero, etc.), by observing that we can remove the booleanity check while preserving soundness.

This is because:

• The first constraint (self - other) * multiplier = is_not_equal guarantees is_not_equal = 0 when self - other = 0. Otherwise, we cannot find a multiplier such that (self - other) * multiplier = 0 * multiplier = is_not_equal != 0
• The second constraint (self - other) * not(is_not_equal) = 0 guarantees is_not_equal = 1 when self - other != 0. Otherwise, not(is_not_equal) = 1 - is_not_equal != 0, which indicates a contradiction self - other = 0.

See also gnark's implementation: https://github.com/Consensys/gnark/blob/870ef4efa8c9fa11a8778821c8376286207ca19b/frontend/cs/r1cs/api.go#L535-L569

Please correct me if I misunderstood something :)

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[Pratyush]

Thanks for the optimization! I left a small nit to add some more explanation of why the booleanity constraint is implicitly enforced.