More `size_hint` and `count` methods #366

Job	Run time
check (msrv)	1m 56s
check (msrv, --no-default-features)	2m 10s
check (msrv, --no-default-features --features use_alloc)	1m 42s
check (stable)	20s
check (stable, --no-default-features)	14s
check (stable, --no-default-features --features use_alloc)	14s
check (stable, --all-targets --all-features)	34s
test	1m 21s
bors build finished	1s
	8m 32s

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