Support "default" dispatchers in ExitNodeDispatcher

[jsiirola]

Fixes # .

Summary/Motivation:

This PR significantly simplifies the definition of the ExitNodeDispatcher lookup tables by allowing a "default" handler for each node type. In particular, this significantly simplifies the construction of derived dispatchers (like the quadratic dispatcher).

Changes proposed in this PR:

• support declaring a "default" exit node handler for each node type
• propagate through the linear and quadratic ExitNodeDispatcher
• update the ExitNodeDispatcher.__missing__ to cleanly handle fallback from argument-specific handlers to the default handler

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