decompression: propagate size_hint when body is identity #1105

Job	Run time
check-docs	44s
cargo-public-api-crates	19s
cargo-hack	1m 16s
check	1m 6s
cargo-deny check (advisories)	20s
cargo-deny check (bans licenses sources)	21s
test-versions (stable)	1m 26s
test-msrv	2m 6s
test-versions (beta)	1m 34s
style	8s
test-versions (nightly)	1m 21s
	10m 41s

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