Inference tip cache is unbounded

[jacobtylerwalls]

See failing pylint primer runs.

My mac shows 20% performance regression running time pylint music21.

bisected to #2158

initial ideas:

• the inference tip cache probably needs to be rewritten as an LRU cache.
• or, we need to hash equivalent InferenceContexts somehow (I started to explore this in Complete cache key for inference tip #2158 but didn't ultimately pursue it)