feat: Add wrapper of BGZF writer

[jemma-nelson]

This patch exposes BGZF writing capabilities. This is my first time writing unsafe code (outside of tutorials), so some extra attention there would be welcomed. :)

One possibly controversial implementation choice I made:
BGZF supports two different versions of "No Compression." The value -2 / wu results in a "bgzf" writer that simply passes the input through without modifying it - the output file should be the exact same as the input. The value 0 / w0 results in a more normal/usual "Uncompressed" setting, where the output is conforming bgzf format, but doesn't try to compress it.

I chose to expose both of these, calling the first one "NoCompression" and the latter "Uncompressed." These names aren't clear in how they differ, so I would be happy for them to be renamed - or, if the situation is too confusing, removing NoCompression.

If accepted, this PR should close #347.

[brainstorm]

Thanks for this PR @jemma-nelson. You even seem to take into account testing all levels except NoCompression... is it because of this issue?: zaeleus/noodles#96

[jemma-nelson]

I was actually unaware of that issue. I'm not sure if it affects us, since I think htslib might be handling that under the hood.

I have a test for the NoCompression level (in which it writes the contents directly, not BGZF encoded) in fn test_write_plain(), but it could probably have a more obvious name. :)