Run a benchmark with an environment.
env is a clonable prototype for the "benchmark environment". Each iteration
receives a freshly-cloned mutable copy of this environment. The time taken to clone the
environment is not included in the results.
Nb: it's very possible that we will end up allocating many (>10,000) copies of
env at the
same time. Probably best to keep it small.
bench and the module docs for more.
bench_env performs a lookup into a big vector in order to get the
environment for that iteration. If your benchmark is memory-intensive then this could, in the
worst case, amount to a systematic cache-miss (ie. this vector would have to be fetched from
DRAM at the start of every iteration). In this case the results could be affected by a hundred
nanoseconds. This is a worst-case scenario however, and I haven't actually been able to trigger
it in practice... but it's good to be aware of the possibility.