Add ability to count opcodes per run

[dantleech]

It would be possible to determine the number of opcodes for a benchmark by dumping the opcodes and counting them.

This could possibly be a new executor, it would be great to be able to combine the results from multiple executors in the same variant (e.g. allow @Assert(mode(variant.time.avg) < 10ms AND variant.opcode.count < 1000)