Merge `Merge/MergeBy/MergeJoinBy` implementations (v2) #421

Job	Run time
test	1m 39s
check format	9s
check (msrv)	3m 27s
check (msrv, --no-default-features)	3m 18s
check (msrv, --no-default-features --features use_alloc)	3m 19s
check (stable)	12s
check (stable, --no-default-features)	7s
check (stable, --no-default-features --features use_alloc)	13s
check (stable, --all-targets --all-features)	39s
bors build finished	3s
	13m 6s

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