Introduce `CountedInput`. a wrapper which counts the number of bytes successfully read #186

Job	Run time
set-image	1s
fmt	1m 1s
clippy	1m 11s
checks	1m 13s
miri (bit-vec)	1m 28s
tests	1m 13s
miri (bytes)	1m 29s
bench-rust-nightly (bit-vec)	1m 6s
miri (generic-array)	1m 29s
bench-rust-nightly (bytes)	1m 6s
miri (arbitrary)	1m 25s
bench-rust-nightly (generic-array)	1m 7s
bench-rust-nightly (derive)	1m 5s
bench-rust-nightly (max-encoded-len)	1m 8s
build-linux-ubuntu-amd64	0s
publish-dry-run	0s
	16m 2s

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