[Core] Adjust FizzMigrate to use Introspection method to work out what it has to do

[SkylarKelty]

• Refactor: Add Table class under Structures
• Refactor: Add DB class under Structures
• At each checkpoint, check live database against what we expect to generate operations
• Get rid of operations stack
• Dont use table comments to version
• Allow users to set comments on tables