Add ElemList

[RavuAlHemio]

This allows enumerating control values on a device.

[diwic]

This looks like a well written patch, thanks! My only thought is that one could avoid that extra call to snd_ctl_elem_list_malloc during initialization.

And maybe add some kind of iterator, maybe one that outputs ElemId but I'm not sure if that's a good idea or not.

Anyhow this patch is good enough to be merged as-is and if you want to write follow-up PRs with my suggestions (or your own) you're welcome to do so :-)

Let me know if you need a release. Thanks!

[RavuAlHemio]

Thanks for the review and merge; I'll contribute additional PRs with your suggestions.