Chapter iteration benchmark

[WaDelma]

Currently the benchmark shown there doesn't assign the return value of Repeat anywhere. This means that it could get optimized away and we'd get useless benchmark results.

There should be at least warning about this or the example should be modified so that such optimization cannot happen. Here is an example how this can apparently be done.

[quii]

Today I learned! It would be good if someone could add a note about this

[deining]

With PR #781 merged, this issue should be closed.