Store CheffeIngredients as pointers
Rename MixingBowl & BakingDish operands
Add initial IngredientOp for CheffeMethodSteps
Rename parameter
Overhaul emitDiagnosticIfIngredientUndefined function

The function now returns a std::pair indicating whether a given
ingredient was undefined or not and a pointer to the CheffeIngredient
(nullptr) if undefined.
Refactor Ingredient dumping
Properly track used Ingredients in CheffeMethodSteps
Test Until Verbed without ingredient
Add NumberOps to Stir & Refrigerate MethodSteps