audit: fix silent metatx merge in comp timelock

[pscott]

• Ensure there are not duplicate MetaTransaction (excluding the salt) by using an array in memory (complexity is O(n^2) but we don't expect the list to be long in the average scenario: and it's better to use an array in memory than a mapping in storage for the "normal" cases)
• Adds a test to ensure it has been fixed properly

Closes #178

[codecov[bot]]

Codecov Report

Merging #212 (4c5c6f1) into main (4027118) will increase coverage by 0.20%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #212      +/-   ##
==========================================
+ Coverage   80.82%   81.03%   +0.20%     
==========================================
  Files          28       28              
  Lines         459      464       +5     
  Branches      111      112       +1     
==========================================
+ Hits          371      376       +5     
  Misses         73       73              
  Partials       15       15              

Impacted Files	Coverage Δ
...tegies/CompTimelockCompatibleExecutionStrategy.sol	82.69% <100.00%> (+1.84%)	:arrow_up: