Implement gzseek for writes

[brian-pane]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 82.50000% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
libz-rs-sys/src/gz.rs	82.50%	7 Missing ⚠️

Flag	Coverage Δ
fuzz-compress	?
fuzz-decompress	?
test-aarch64-apple-darwin	89.11% <77.50%> (+1.64%)	⬆️
test-x86_64-apple-darwin	85.64% <77.50%> (-0.11%)	⬇️
test-x86_64-unknown-linux-gnu	87.06% <72.50%> (+0.05%)	⬆️

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Files with missing lines	Coverage Δ
libz-rs-sys/src/gz.rs	93.12% <82.50%> (+3.39%)	⬆️

... and 6 files with indirect coverage changes

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[folkertdev]

this is almost certainly unsound. Would https://doc.rust-lang.org/std/primitive.pointer.html#method.write_bytes work?