gzip: make header functions unsafe and document preconditions #944

Job	Run time
Smoke-test fuzzing targets	2m 1s
Clippy	24s
Build and test (stable, ubuntu-latest, x86_64-unknown-linux-gnu)	55s
Smoke-test fuzzing targets (--no-default-features --features="c-allocator")	55s
QEMU build & test (stable, ubuntu-latest, s390x-unknown-linux-gnu, s390x-linux-gnu-gcc)	43s
Clippy (--no-default-features --features="c-allocator")	25s
Build and test (msrv, ubuntu-latest, x86_64-unknown-linux-gnu)	57s
Smoke-test fuzzing targets (--no-default-features --features="rust-allocator")	50s
Clippy (--no-default-features --features="rust-allocator")	18s
QEMU build & test (stable, ubuntu-latest, i686-unknown-linux-gnu, i686-linux-gnu-gcc)	47s
Build and test (beta, ubuntu-latest, x86_64-unknown-linux-gnu)	56s
Build and test (stable, macos-latest, x86_64-apple-darwin)	1m 5s
Smoke-test fuzzing targets (stable, macos-14, aarch64-apple-darwin)	2m 13s
dynamic library	22s
Build and test (stable, macos-14, aarch64-apple-darwin)	1m 5s
Miri	2m 50s
Build and test (stable-x86_64-gnu, windows-2022, x86_64-pc-windows-gnu)	2m 1s
wasm32	57s
	19m 44s

Provide feedback