Use internal functions for querying nativeTokenAddress() in L1StandardBridge and L1CrossDomainMessenger

[nguyenzung]

Current implementation on L1StandardBridge and L1CrossDomainMessenger

    function nativeTokenAddress() public view returns (address) {
        return systemConfig.nativeTokenAddress();
    }

Improvement:

    function nativeTokenAddress() public view returns (address) {
        return _nativeToken();
    }

       function _nativeToken() internal view returns (address) {
        return systemConfig.nativeTokenAddress();
    }

Using internal function instead of nativeTokenAddress() in the same contract helps us saving gas because there is no more sub-context created calling to itself

[theo-learner]

Note: this issue is self-reported by protocol team (Tokamak Network)

[sherlock-admin2]

The protocol team fixed this issue in the following PRs/commits: https://github.com/tokamak-network/tokamak-thanos/pull/289