Significant speedup on EC-DSA verfification

Significant speedup on EC-DSA verfification: Accelerates ECC point multiplication and modular reduction by removing constant-time bottlenecks (not needed in this process as verification only deals with public information). Slightly increased code size (same lines of expected result for other ECC optimizations).

Fix to make ECC Key Generation FIPS 186-4 compliant
- Previous version used a simplified method

Additional check in ecc_valid_public_key
- Also check that the public key is not the generator of P-256

Fix in double_bytes function
- Previous version used by Colin O'Flynn to show power analysis threat

Change in HMAC API:
- API now ensures it erases secrets before exiting

Added Authors file
- Clarifies contacts for responsible disclosure