Recently the library added a new ranged-for variant of the KeepRunning
loop that is much faster. For this reason it should be preferred in all
new code.

Because a library, its documentation, and its tests should all embody
the best practices of using the library, this patch changes all but a
few usages of KeepRunning() into for (auto _ : state).

The remaining usages in the tests and documentation persist only
to document and test behavior that is different between the two formulations.

Also note that because the range-for loop requires C++11, the KeepRunning
variant has not been deprecated at this time.