Rename `group`/`group_by` arguments into `by` #470

Job	Run time
R-CMD-check / macOS-latest (release)	7m 23s
R-CMD-check / windows-latest (devel)	19m 2s
R-CMD-check / windows-latest (release)	20m 13s
R-CMD-check / windows-latest (oldrel)	20m 46s
R-CMD-check / windows-latest (4.1)	4m 5s
R-CMD-check / ubuntu-latest (devel)	14m 34s
R-CMD-check / ubuntu-latest (release)	9m 53s
R-CMD-check / ubuntu-latest (4.3)	25m 37s
R-CMD-check / ubuntu-latest (4.2)	51s
R-CMD-check / ubuntu-latest (4.1)	51s
R-CMD-check / ubuntu-latest (4.0)	46s
R-CMD-check / ubuntu-latest (3.6)	1m 2s
	2h 5m 3s

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