POC: Opcode counter

[dantleech]

This PR experiments with adding an opcode executor which provides a count of the opcodes executed during the benchmark.

TL;DR: It should be possible to provide a count of the opcodes for the subject only.

Counting Opcodes

• It seems not possible to enable the opcode debugging during execution (e.g. to capture only the subject's opcodes)
• It would be possible to first execute the script without running the subject, then again with the subject, and subtract the first from the second.

Including Opcodes alongside the regular results

Would be possible to add a new CompositeExecutor which ran a set of executors. I.e. to execute the opcode executor and the regular executor.

Pre and post optimisation

By using the CompositeExecutor it would be possible to run two instances of the Opcode executor, one for unoptimised and one with optimised results.

Debugging Opcodes

It would also be good to add a convenience command to debug the opcodes. However, I cannot see a way to separate the signal from the noise. The opcodes are not presented in the order that would make this possible.