Introducing BoundTreeVisitor and BoundTreeWalker

[ChrisKXu]

Summary

This PR introduces BoundTreeVisitor and BoundTreeWalker patterns to all the bound nodes.
This PR also converted ControlGraphAnalyzer to inherit from BoundTreeWalker.

Detail

Similar to roslyn, we use the Visitor Pattern to traverse the BoundTree and the nodes. This requires that

• All sealed BoundNode types needs to implement Accept(T) method. (This could even be generated in the future)
• We have a corresponding virtual Visit* method present in the BoundTreeVisitor base class.

The benefit to this approach is that we don't need a centralized Visit method with switch statements for every single BoundNode types.

BoundTreeVisitor

BoundTreeVisitor class is the base class which has all the Visit* methods return null. It provides boilerplate code for children classes to extend. Analyzers and rewriters should not directly inherit from this class and instead use BoundTreeWalker or BoundTreeRewriter (not implemented yet).

BoundTreeWalker

BoundTreeWalker class inherits from BoundTreeVisitor and provides basic depth-first traversal functionality. All nodes are examined and are not modified. Analyzers that don't modify the nodes should inherit from this class.

BoundTreeRewriter (not in this PR)

BoundTreeRewriter class also inherits from BoundTreeVisitor. It is similar to BoundTreeWalker but nodes can be updated (note, BoundNodes are immutable by design so here "updates" to the node actually creates a new node and replace the old one). The detailed implementation will be provided in future PRs.