Detect changes to the AppStorage structure

[hiddentao]

Detecting changes to the AppStorage structure would be great as a way of guarding against diamond corruption.

See https://eip2535diamonds.substack.com/p/example-of-a-diamond-upgrade for more info.

Specifically:

• All new storage members must be appended to the existing App storage struct.

Gemforge can optionally support checking for adherence to these rules. The proposal:

• New diamond.storage config key which points to:

{ 
   type: "AppStorage",  
   config: {
      file: "PathToFileContainingAppStorageStruct.sol" 
      struct: "NameOfAppStorageStruct" 
   }
}

• Gemforge will complain if existing items are changed (even by name)
• Gemforge will complain if the struct/file cannot be found.
• All checks will be done during the build, when GemForge parses the code.