Gas Optimizations

[code423n4]

• The check require(amount <= mintableSupply, "INVALID_AMOUNT"); in VariableSupplyERC20Token.mint is unnecessary, because the subtraction on the next line will underflow anyways if this is not true.
• Decrementing the variable mintableSupply in VariableSupplyERC20Token.mint is not necessary. The remaining mintable supply could always be calculated based on the initialSupply_ (if it would be stored), the maximum supply and the already minted tokens (standard ERC20 variable _totalSupply). The remaining supply is then maxSupply_ - _totalSupply + initialSupply_.
• In VTVLVesting.sol:353, the loop iteration can be marked as unchecked because an overflow is not possible (as the iterator is bounded) and i++ can be replaced with ++i.
• Custom errors could be used instead of string error reasons to save gas.