Gas Optimizations

[code423n4]

Title: Using delete statement to empty rewardsAccrued can save gas

Proof of Concept:
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/FlywheelCore.sol#L123

Recommended Mitigation Steps:
Change to:

	delete rewardsAccrued[user];

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Title: Using != is more gas efficient

Proof of Concept:
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/FlywheelCore.sol#L167
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/FlywheelCore.sol#L218

Recommended Mitigation Steps:
Change to:

	if (oldRewardBalance != 0) {

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Title: Using storage to declare Struct variable inside function

Proof of Concept:
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/FlywheelCore.sol#L85
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/FlywheelCore.sol#L106

Recommended Mitigation Steps:
instead of caching RewardsState to memory. read it directly from storage.

	RewardsState storage state = strategyState[strategy];

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Title: Using calldata on struct parameter

Proof of Concept:
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/FlywheelCore.sol#L210
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/FlywheelCore.sol#L241

Recommended Mitigation Steps:
Using calldata to store struct data type can save gas

	function accrueStrategy(ERC20 strategy, RewardsState calldata state)

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Title: Using unchecked to calculate can save gas

Proof of Concept:
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/rewards/FlywheelStaticRewards.sol#L60

Recommended Mitigation Steps:
rewards.rewardsEndTimestamp is checked that it won't > than lastUpdatedTimestamp

unchecked{
	elapsed = rewards.rewardsEndTimestamp - lastUpdatedTimestamp;
}

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Title: Using > is cheaper than >=

Proof of Concept:
https://github.com/Rari-Capital/solmate/blob/9f16db2144cc9a7e2ffc5588d4bf0b66784283bd/src/tokens/ERC20.sol#L125

Recommended Mitigation Steps:
just use > can save gas
Change to:

	require(deadline > block.timestamp, "PERMIT_DEADLINE_EXPIRED");

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Title: Using immutable can save gas

Proof of Concept:
https://github.com/Rari-Capital/solmate/blob/9f16db2144cc9a7e2ffc5588d4bf0b66784283bd/src/tokens/ERC20.sol#L23

Recommended Mitigation Steps:
use immutable to declare variable which set once in constructor

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Title: Using multiple require instead && can save gas

Proof of Concept:
https://github.com/Rari-Capital/solmate/blob/9f16db2144cc9a7e2ffc5588d4bf0b66784283bd/src/tokens/ERC20.sol#L154

Recommended Mitigation Steps:
Change to:

	require(recoveredAddress != address(0), "INVALID_SIGNER");
	require(recoveredAddress == owner, "INVALID_SIGNER");

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Title: unnecessary value set. the default value of uint is 0.

Proof of Concept:
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/token/ERC20Gauges.sol#L134
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/token/ERC20Gauges.sol#L184
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/token/ERC20Gauges.sol#L307
https://github.com/fei-protocol/flywheel-v2/blob/77bfadf388db25cf5917d39cd9c0ad920f404aad/src/token/ERC20Gauges.sol#L384

Recommended Mitigation Steps:
remove 0 value can save gas

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