Meta issue: 4.3

[Amxx]

• Add an interface folder that lists common interfaces #2517
• Optimize EOA signature verification #2661
• Governor #2672
• Use private variable instead of immutable to allow calls to _msgSender in constructor. #2754
• Gas optimization on average function of Math.sol #2757
• Add values() functions to EnumerableSets #2768
• Improve Governor #2794
• Avoid safe math operations in BitMap #2797
• Read optional compiler version from args/envvars #2810
• Add Governor guide #2811
• Use both IGovernor (interface) and AGovernor (abstract contract) #2812

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• Add API docs for new interfaces.
• Turn coverage back on for supportsInterface.