Perform code review for vrf-solidity

[girazoki]

This issue aims to analyze (with the proper auditing tools) the potential vunerabilities that the library might have.

[girazoki]

The tools utilized were:

MythX:

• SWC-103 (Low): Floating Pragma - non-fixed version of solidity.

Mythril:

• The analysis was completed successfully. No issues were detected.

Slither:

• Functions declared as view but contain assembly code. However, the assembly code does not modify state variables so the visibility is correct.
• Non-fixed version of solidity.

Smartcheck:

• Functions declared as view but contain assembly code. However, the assembly code does not modify state variables so the visibility is correct.
• Revert inside if statements is discouraged, use require.
• Non-fixed version of solidity.
• Implicit visibility level. However the visibility is correct as those variables need to be internal.

Manticore:

• No issues found.