rs-soroban-sdk: `Fr` scalar field equality comparison bypasses modular reduction

Security Advisory: Incorrect Equality for Fr Scalar Field Types BN254, BLS12-381 Summary Missing modular reduction in Fr causes incorrect equality comparisons for BN254 and BLS12-381 types in soroban-sdk. Impact The Fr scalar field types for BN254 and BLS12-381 in soroban-sdk compared values usin...