more on Timer.

Having looked at it, I don't think yesterday's change are useful enough as is. These changes make it so the data collected by a Timer are absolute time points, and the `operator SEXP()` transform these steps into time since the origin.

In conjunction with `Timer::get_timers` which makes several `Timer` with the same origin, the new ctor `Timer(nanotime_t)` and the method `Timer::origin()` it makes something that is much more useful for tracking what goes on with multiple threads. 

Note that if we want the data as before, we can use `diff` on the R side results. This will be more accurate anyway because Timer does less so is less of an influence.