Enable grouped events

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@gz, thanks for the good work here! I was hoping to use perfcnt to measure grouped events but I see that the ReadRecord in src/linux/mod.rs does not seem to support this. The ReadRecord in src/linux/perf_format.rs does seem to have the support but it is commented out in the EventData enum. What did you find that prevented using the more flexible record format?

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Oh, the other thing to make this work would be to expose the group leader's fd in some way in PerfCounter, perhaps a PerfCounter::fd() -> Result<isize, ...> or the like so that we can then use in PerfCounterBuilderLinux::set_group? @iximeow describes this in more detail here.