Use BlockCipher.blockSize as a result length in randomIV helper

[evnik]

Checklist:

• [ ] Correct file headers (see CONTRIBUTING.md) (not needed).
• [x] Formatted with SwiftFormat.
• [x] Tests added.

Changes proposed in this pull request:

Use BlockCipher.blockSize as a result length in randomIV helper. This should help to avoid two mistakes when randomIV helper is used:

• Passing of invalid array length for IV generation.
• Usage of randomIV helper for a key generation. Because it uses Swift.SystemRandomNumberGenerator, depending on the platform the result might be not cryptographically secure to be used as a key. Security.SecRandomCopyBytes should be preferred to generate a key on Apple platforms.

[evnik]

@krzyzanowskim any feedback for this?