Update ExitNodeDispatcher to support subclasses, improved customization

[jsiirola]

Fixes # .

Summary/Motivation:

This PR updates the ExitNodeDispatcher to better support extensibility:

• Automatically handle derived classes using known base class handlers
• Provide a hook to change the behavior for unknown classes

Note that this does not address #3111 (although these changes are compatible with adding an etype class attribute).

Changes proposed in this PR:

• Support automatic handling of derived expression classes using known base class handlers
• Provide unexpected_expression_type() method so derived dispatchers can change the behavior for unbknown expression types (enables simpler implementation of bypass unrecognized expression types in compute_bounds_on_expr #3086)
• Add tests

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