This is a long-time coming massive simplification of internals that results in better design. Once done, this will make building new features and fixing any command-related bugs easier.

TODO

• Make commands use datasets instead of relations
• Make building commands work again
• Make compiling commands work again
• Make commands use schemas
  • Add schema option
  • Infer input_proc from schema
  • Infer output_proc from schema (this is now used in rom-sql but it should be in the core)
• Fix command graphs
• Fix lazy commands
• Update rom-sql too (this is also needed to make specs pass over here) - see Decouple Command from Relation rom-sql#387

Closes #605