More `size_hint` and `count` methods #365

Job	Run time
test	1m 18s
check (msrv)	1m 40s
check (msrv, --no-default-features)	2m 1s
check (msrv, --no-default-features --features use_alloc)	1m 52s
check (stable)	8s
check (stable, --no-default-features)	5s
check (stable, --no-default-features --features use_alloc)	9s
check (stable, --all-targets --all-features)	20s
bors build finished	1s
	7m 34s

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