Index CoverageMap and SignatureMap by codehash (for performance)

[samalws-tob]

This PR makes it so the CoverageMap and SignatureMap are indexed by contracts' codehashes, rather than their bytecode metadata. As a result:

• This contract ran 60% faster on my computer, producing the same output as before.
• This contract ran 18% faster on my computer, producing the same output as before.

Additional changes:

• Removes the bytecode metadata cache, since this is no longer needed
• Adds a codehash cache. This is because we need to index maps by a contract's "real" / "original" / "compile-time" codehash, rather than its .codehash property (which can be different if Solidity's immutables feature, which changes bytecode during compile time, is used). The real codehash is found using hevm's findSrc function, and the results are cached in the codehash map
• Adds an additional test for immutables.
• Uses codeContract rather than contract to determine the current contract while keeping track of coverage (after talking this over with @arcz ). Beforehand, this was done in some places but not in others. I believe this should fix some delegate call behavior.

[arcz]

This looks great. The immutable.sol seems to fail because contractAList in genTx is empty, so it looks like a bug with SignatureMap.

[arcz]

Amazing work!