Re-wrote the `MockLido` contract to store its balances as shares

[jalextowle]

Resolved Issues

Fixes: #799.

Description

After some issues were encountered with the agent0 fuzzing that used the MockLido vault, it became apparent that the current methodology used by the MockLido vault wasn't acceptable. This PR rewrites the mock yield source so that we can continue fuzzing the stETH integration.

Review Checklists

Please check each item before approving the pull request. While going through the checklist, it is recommended to leave comments on items that are referenced in the checklist to make sure that they are reviewed. If there are multiple reviewers, copy the checklists into sections titled ## [Reviewer Name]. If the PR doesn't touch Solidity, the corresponding checklist can be removed.

[[Reviewer Name]]

• [ ] Tokens
  • [ ] Do all approve calls use forceApprove?
  • [ ] Do all transfer calls use safeTransfer?
  • [ ] Do all transferFrom calls use msg.sender as the from address?
    • [ ] If not, is the function access restricted to prevent unauthorized token spend?
• [ ] Low-level calls (call, delegatecall, staticcall, transfer, send)
  • [ ] Is the returned success boolean checked to handle failed calls?
  • [ ] If using delegatecall, are there strict access controls on the addresses that can be called? It shouldn't be possible to delegatecall arbitrary addresses, so the list of possible targets should either be immutable or tightly controlled by an admin.
• [ ] Reentrancy
  • [ ] Are functions that make external calls or transfer ether marked as nonReentrant?
    • [ ] If not, is there documentation that explains why reentrancy is not a concern or how it's mitigated?
• [ ] Gas Optimizations
  • [ ] Is the logic as simple as possible?
  • [ ] Are the storage values that are used repeatedly cached in stack or memory variables?
  • [ ] If loops are used, are there guards in place to avoid out-of-gas issues?
• [ ] Visibility
  • [ ] Are all payable functions restricted to avoid stuck ether?
• [ ] Math
  • [ ] Is all of the arithmetic checked or guarded by if-statements that will catch underflows?
  • [ ] If Safe functions are altered, are potential underflows and overflows caught so that a failure flag can be thrown?
  • [ ] Are all of the rounding directions clearly documented?
• [ ] Testing
  • [ ] Are there new or updated unit or integration tests?
  • [ ] Do the tests cover the happy paths?
  • [ ] Do the tests cover the unhappy paths?
  • [ ] Are there an adequate number of fuzz tests to ensure that we are covering the full input space?

[coveralls]

Pull Request Test Coverage Report for Build 9718140985

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.667%

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Totals
Change from base Build 9690425513:	0.0%
Covered Lines:	1984
Relevant Lines:	2141

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💛 - Coveralls

[github-actions[bot]]

Hyperdrive Gas Benchmark

Benchmark suite	Current: 016dd0b04d67fccf43b633f58b6c2c5c24aa9092	Previous: e84bb23edd4b8d4b6ee42dac6c225bde9df2de3c	Deviation	Status
addLiquidity: min	33937 gas	33937 gas	0%	🟰
addLiquidity: avg	156552 gas	156075 gas	0.3056%	🚨
addLiquidity: max	429262 gas	429262 gas	0%	🟰
checkpoint: min	40316 gas	40316 gas	0%	🟰
checkpoint: avg	144763 gas	144745 gas	0.0124%	🚨
checkpoint: max	255940 gas	255940 gas	0%	🟰
closeLong: min	31361 gas	31361 gas	0%	🟰
closeLong: avg	135570 gas	135881 gas	-0.2289%	✅
closeLong: max	2539376 gas	2539376 gas	0%	🟰
closeShort: min	31327 gas	31327 gas	0%	🟰
closeShort: avg	131201 gas	131156 gas	0.0343%	🚨
closeShort: max	271117 gas	271117 gas	0%	🟰
initialize: min	31349 gas	31349 gas	0%	🟰
initialize: avg	333412 gas	333484 gas	-0.0216%	✅
initialize: max	399746 gas	399746 gas	0%	🟰
openLong: min	33370 gas	33370 gas	0%	🟰
openLong: avg	172967 gas	172902 gas	0.0376%	🚨
openLong: max	333831 gas	333831 gas	0%	🟰
openShort: min	33936 gas	33936 gas	0%	🟰
openShort: avg	168219 gas	168373 gas	-0.0915%	✅
openShort: max	415052 gas	414680 gas	0.0897%	🚨
redeemWithdrawalShares: min	31251 gas	31251 gas	0%	🟰
redeemWithdrawalShares: avg	74389 gas	75443 gas	-1.3971%	✅
redeemWithdrawalShares: max	305459 gas	305459 gas	0%	🟰
removeLiquidity: min	31301 gas	31301 gas	0%	🟰
removeLiquidity: avg	215576 gas	215556 gas	0.0093%	🚨
removeLiquidity: max	403641 gas	403858 gas	-0.0537%	✅

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