Add per-epoch setup() fluent API

[l0rinc]

Add Bench::setup(...).run(...) for untimed per-epoch setup.
The existing run() now delegates to runImpl() with an empty setup lambda.

Tests verify setup is excluded from timing (50ms sleep → ~0 measured) and runs once before each epoch's iterations (S-RRR-S-RRR pattern).

Fixes: #130
Alternative to: #135

[urcheon]

For symmetry you should also have teardown, and I can think of use cases where teardown would be helpful.

The only problem I foresee is that with fluent API it becomes a little unintuitive with

Bench::setup(...).teardown(...).run(...)

[l0rinc]

We can do that in a follow-up, but a setup should already fix most cases since we usually need the benchmarking conditions to be met - and tearing it down can either be done in the next setup run or when existing the benchmark.