Error on non scalar aggregates regardless of fill

[Mukulyadav2004]

closes #6629

In dcast, fun.aggregate must return length 1. Previously, with fill != NULL, dcast warned but still produced an undefined result #6032. Plan is to turn that into an error

So in this PR
Dcast breaking change(fcast.R)

• fun.aggregate must return length 1. Previously, with fill != NULL, dcast warned but still produced an undefined result (revdep failure from dcast error if fun.aggregate returns length>1 #6032). Now this unconditionally errors and the warn branch is removed.

@tdhock @joshhwuu @Anirban166 please review.

[codecov]

Codecov Report

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Additional details and impacted files

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##             master    #7260   +/-   ##
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  Coverage          ?   98.80%           
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  Files             ?       81           
  Lines             ?    15250           
  Branches          ?        0           
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  Hits              ?    15067           
  Misses            ?      183           
  Partials          ?        0           

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[github-actions]

• HEAD=6629_issue slower P<0.001 for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit d000179

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 52 seconds
Installing different package versions	10 minutes and 4 seconds
Running and plotting the test cases	2 minutes and 37 seconds

[tdhock]

sorry I merged too quickly. please submit another pr with a news item

[Mukulyadav2004]

Sure