Deprecate `group_by` in favor of `chunk_by` #756

Job	Run time
doc	9s
msrv	48s
check	20s
check format	11s
semver-checks	19s
check (--no-default-features)	7s
test	59s
check (--no-default-features --features use_alloc)	9s
check (--all-targets --all-features)	28s
All checks succeeded	1s
	3m 31s

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