Flaw in `FieldVar::mul_by_inverse` allows unsound R1CS constraint systems

Versions 0.2.0 to 0.3.0 of ark-r1cs-std did not enforce any constraints in the FieldVar::mulbyinverse method, allowing a malicious prover to produce an unsound proof that passes all verifier checks. This method was used primarily in scalar multiplication for shortweierstrass::ProjectiveVar. This...