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::mul_by_inverse method, allowing a malicious prover to produce an unsound proof that passes all verifier checks.
This method was used primarily in scalar multiplication for short_weierstrass::ProjectiveVar.

This bug was fixed in commit 47ddbaa, and was released as part of version 0.3.1 on crates.io.