Factor out tracing and optimization heuristics into a single "policy" object.

[markshannon]

The SAI, tracer and optimizer all contain a number of heuristics about when to optimize, how to link the optimized code and when to backoff, etc.

Almost all of these heuristics are just numbers.
We should group them together into a single "policy" object, improving consistency and enabling experimentation.

[Fidget-Spinner]

This sounds rather promising. It would also enable per-system configurations, like pypy which would be great.