Add profiling counters about mijit HOT 3 OPEN

Should probably do #26 first.

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Plan:

• Add profiling counters to retire transitions.
• Compute statistics.
• Compute probabilities.
• Throw a Mijit exception when a counter reaches a threshold.

from mijit.

Slight change of plan. We have worked out a cheaper and less accurate way of profiling execution which we think will be good enough.

Instead of an in-memory profiling counter per basic block, we propose to have a single in-register counter that is shared by all basic blocks: effectively a sort of timer. When it overflows (every 10,000 basic blocks, say?) we interrupt the compiled code and record where it happened. If ever we see n interrupts have occurred in the same basic block (n might be as small as 2) we can treat it as hot code.

This new design simplifies the compiled code at the expense of moving profiling information to the Rust data structures, which is arguably a win. But the main motivation for the new idea is to avoid accessing in-memory profiling counters while running hot code.

I'm going to keep this issue open, as it is mostly still relevant.

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