Improve efficiency by using immutable in TreasuryVester
The `TreasuryVester` contract when deployed has a few fixed storage variables.\\n```\\ngtc = gtc\\_;\\n```\\n\\n```\\nvestingAmount = vestingAmount\\_;\\nvestingBegin = vestingBegin\\_;\\nvestingCliff = vestingCliff\\_;\\nvestingEnd = vestingEnd\\_;\\n```\\n\\nThese storage variables are defined in the contract.\\n```\\naddress public gtc;\\n```\\n\\n```\\nuint public vestingAmount;\\nuint public vestingBegin;\\nuint public vestingCliff;\\nuint public vestingEnd;\\n```\\n\\nBut they are never changed.
Resolution\\nFixed in gitcoinco/governance#5\\nConsider setting storage variables as `immutable` type for a considerable gas improvement.
```\\ngtc = gtc\\_;\\n```\\n