Suggestion: A few minor gas optimizations for the example contracts

[ebgd]

Hi team,

First of all, thanks for this excellent Foundry starter kit! It's a great resource for the community.

While looking through the example contracts (VRFConsumerV2.sol and KeepersCounter.sol), I noticed a couple of small gas optimization opportunities that could be nice additions to make the templates even more efficient.

1. Make s_owner immutable in VRFConsumerV2.sol

The s_owner state variable is set in the constructor and never changed afterwards. Marking it as immutable would save significant gas on every read by replacing an expensive SLOAD opcode with a cheap PUSH of the value directly from the contract's bytecode.

Suggestion:

- address public s_owner;
+ address public immutable s_owner;

Approximate Gas Savings: This typically saves ~2000 gas each time the onlyOwner modifier is checked, as it avoids a storage read.

2. Use Custom Errors in KeepersCounter.sol

The require statement in performUpkeep uses a string message ("Time interval not met"). Since Solidity 0.8.4, using custom errors is more gas-efficient both at deployment and during runtime reverts.

Suggestion:

// At the contract level, above the constructor
+ error UpkeepNotNeeded();

// In the performUpkeep() function
- require(upkeepNeeded, "Time interval not met");
+ if (!upkeepNeeded) {
+     revert UpkeepNotNeeded();
+ }

Approximate Gas Savings: This provides two benefits:

1. Deployment: A small saving by not storing the 23-byte revert string in the contract bytecode.
2. Runtime (on revert): A more significant saving of ~250-400 gas each time the transaction reverts. This is because a custom error avoids the memory allocation and copying operations required for a revert string.

These are just small suggestions to make the starter kit even better. Thanks again for the great work!