Add object and key to visitor callbacks.

[TwitchBronBron]

• Provides the parent object and the key used to fetch the item from the parent object in AST walk visitor callbacks. This makes AST editing a little easier when it's desired to actually remove the element from the parent AST.
• update all AST walk methods to support adding/removing items during the walk cycle without messing up the iteration.

Also
Resolves #477