`TupleCombinations`: `size_hint` and `count` #458

Job	Run time
check format	9s
test	1m 25s
check	16s
msrv	1m 29s
check (--no-default-features)	18s
check (--no-default-features --features use_alloc)	9s
check (--all-targets --all-features)	28s
bors build finished	1s
	4m 15s

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