Audit and Optimize ERC20 and ERC721

[Abeeujah]

This pull request includes a comprehensive audit and optimization of the ERC20 and ERC721 smart contracts. The primary goal is to enhance security, improve efficiency, and ensure adherence to best practices for Solidity smart contract development.

Changes Made:

ERC20 Contract:

Security Enhancements:

• Introduced custom errors (InsufficientBalance, InvalidRecipient, Overflow, Underflow) to reduce gas costs compared to require statements.
• Added an onlyOwner modifier for minting and burning functions to restrict access to these sensitive operations.
• Updated the changeOwner function to emit an Owned event, ensuring transparency in ownership changes.
• Ensured that critical functions like _mint, _burn, and transfer operations correctly handle edge cases such as zero addresses and token existence.

Gas Optimization:

• Implemented unchecked blocks where applicable to save on gas costs.
• Introduced custom errors for common failure cases to optimize gas usage.

Event Emissions:

Ensured that all state-changing functions properly emit events (Transfer, Approval, Owned).
ERC721 Contract:

Code Quality Improvements:

• Refactored functions to improve readability and maintainability.
• Ensured consistent use of modifiers and error handling throughout the contract.

NatSpec Documentation:

• Added comprehensive NatSpec comments to both ERC20 and ERC721 contracts to provide clear, developer-friendly documentation.
• Detailed explanations of function parameters, return values, and events to improve contract usability and understanding.

Impact:

These changes improve the security, efficiency, and maintainability of the ERC20 and ERC721 contracts. The NatSpec documentation also enhance the developer experience by providing clear and concise information on contract functionality.

Testing:

The contracts were compiled and deployed using the remix interface and all functions were checked extensively, including checking for edge cases.

[princeibs]

Loop hole in function: It does not check whether the from address has enough tokens to be burnt from