Minor C++17 cleanup of groupby.cu: structured bindings, more concise lambda, etc

[codereport]

Due to the fact that my soul is slowly melting away due to my current internal PR I am working on, I set up this small cleanup in hopes of getting a dopamine or endorphin hit.

[rgsl888prabhu]

looks good, apart from thing failing with rmm.

[davidwendt]

I think the PR description somehow ends up in the changelog so you may want to consider replacing it with something more descriptive.

[codecov]

Codecov Report

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@@               Coverage Diff               @@
##             branch-21.10    #9193   +/-   ##
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  Coverage                ?   10.77%           
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  Files                   ?      115           
  Lines                   ?    19138           
  Branches                ?        0           
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  Hits                    ?     2062           
  Misses                  ?    17076           
  Partials                ?        0           

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[codereport]

@gpucibot merge