opt(reentrancy-guard): using transient storage to optimize reentrancy guard

[SocksNFlops]

Changes:

• Reimplemented ReentrancyGuard.sol to leverage transient storage
• Open to feedback & suggestions to improve PR

Tests:

• Outperforms everywhere except slightly in the allow-reentrancy test.

Before	After
[PASS] testGuardValueReset() (gas: 36965)	[PASS] testGuardValueReset() (gas: 32474)
[PASS] testProtectedFunctionPreventsReentrancy() (gas: 36096)	[PASS] testProtectedFunctionPreventsReentrancy() (gas: 31268)
[PASS] testUnprotectedFunctionAllowsReentrancy() (gas: 32284)	[PASS] testUnprotectedFunctionAllowsReentrancy() (gas: 32325)

Reviewers:

@z0r0z