Fix Int64 sign checks and mod

[mitschabaude]

Security fixes on Int64 which are enabled when switching to "v2" versions of some methods.

The problem that this attempts to solve:

Int64s represent signed integers as a sign: Sign and magnitude: UInt64. This allows for many efficient implementations. However, it also means that 0 has two representations: +0 and -0.
This leads to ambiguity in the isPositive() function, which ignores the magnitude.
[...]
A malicious prover may choose to use either positive or negative zero.
[...]
For example, Int64.mod() uses the expression Provable.if(this.isPositive(), rest, y_.value.sub(rest)) to select between rest and y_ - rest when computing the remainder modulo y_. By choosing this to -0, an attacker may output y_ instead of 0.

• isPositive() is deprecated, because it is both misleadingly named and incorrect
  • returns true for +0 and false for -0
  • depending on whether you interpret it as "> 0" or ">= 0", that result is wrong for either +0 or for -0.
  • the new isPositiveV2() avoids the ambiguity by checking "> 0", returns false for all versions of 0
• mod() is deprecated, because it is wrong on -0 inputs
  • modV2() is correct
• There's also a new "v2" version of check(), which fixes the root of the problem: That 0 can be represented in two different ways. It can only be enabled when upgrading to v2, because it breaks many circuits

[Trivo25]

should we add this part to the changelog as well?

[mitschabaude]

ok we can do that