More `size_hint` and `count` methods #364

Job	Run time
test	1m 15s
check (msrv)	2m 1s
check (msrv, --no-default-features)	1m 44s
check (msrv, --no-default-features --features use_alloc)	1m 39s
check (stable)	10s
check (stable, --no-default-features)	7s
check (stable, --no-default-features --features use_alloc)	11s
check (stable, --all-targets --all-features)	19s
bors build finished	1s
	7m 27s

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