perf: perform a garbage collection before each iteration Currently, no explicit garbage collection is performed when running the microbenchmarks in `hg perf`. I think this is wrong because garbage collection can have a significant impact on execution times. And, if gc is triggered via the default heuristics, it will fire effectively randomly during subsequent benchmark iterations due to variable amount of garbage left over from previous runs. Running a gc before invoking the measured function will help ensure state is more consistent across all iterations.