Implement gzseek for reads, as well as gzrewind

[brian-pane]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 80.70175% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
libz-rs-sys/src/gz.rs	80.70%	22 Missing ⚠️

Flag	Coverage Δ
fuzz-compress	?
fuzz-decompress	?
test-aarch64-apple-darwin	87.47% <79.82%> (-1.02%)	⬇️
test-x86_64-apple-darwin	85.76% <80.70%> (-0.14%)	⬇️
test-x86_64-unknown-linux-gnu	87.00% <16.66%> (-0.70%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
libz-rs-sys/src/gz.rs	89.73% <80.70%> (-2.56%)	⬇️

... and 4 files with indirect coverage changes

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[brian-pane]

The remaining test error is because I'm checking for expected character sequences at specific offsets in a text file included in the project. Git in the Windows CI environment translates newlines to CRLF when it checks out that file. I'll need to rework the test case to use one of the binary test-data files instead.

[brian-pane]

I only included the read paths for gzseek because this PR is kind of big already. I'm planning to add the write support as a separate PR in the future.