Freeze contract code in preparation for ChainSecurity audit

This pre-release creates a frozen commit for ChainSecurity to begin auditing Comet.sol. We will continue to modify tests, supporting code and docs on git branches. We do not intend to modify the contract further, although we have a couple of decisions still to make about merging open branches of development. These are mainly in regards to how we deploy/govern the contract (i.e. Configurator and Pre-packing). We are also considering an interest rate model change, which we would welcome feedback on, but which should not substantially change the scope of the audit. Finally, we are using Solidity viaIR flag to reduce contract size, but have not merged yet due to it currently breaking Hardhat coverage.