combinations: `count` and `size_hint` #373

Job	Run time
check (msrv)	1m 58s
check (msrv, --no-default-features)	1m 57s
check (msrv, --no-default-features --features use_alloc)	2m 28s
check (stable)	8s
check (stable, --no-default-features)	12s
check (stable, --no-default-features --features use_alloc)	7s
check (stable, --all-targets --all-features)	26s
test	1m 19s
bors build finished	1s
	8m 36s

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