feat: make GzBuilder::filename and GzBuilder::comment results

[CosminPerRam]

To handle possible panics with user-povided input, return a Result on GzBuilder::comment and GzBuilder::filename.

We build a CString, and if it fails, it returns a NulError which converts to io::Error as an InvalidInput.

Doing so will cause breaking api changes, this is completely debatable if its worth it or not, so this PR is more like a proposal.

For more context:
The unwraps were added here: 6430b38, back in 2015, which makes them 10-years old now.

[jongiddy]

Changing the public API is unlikely to happen. The panic condition is well-documented and can be tested beforehand if the data is provided by the user.

There is a possibility that new Result-returning methods could be added, such as with_filename but, if this is done, it would be worth discussing which type is most appropriate. Passing a CString directly would work and avoid any errors. But arguably a PathBuf is a more appropriate type for a filename. The string is encoded in ISO 8859-1 so enforcing this might also be desirable.

[Byron]

Thanks a lot for proposing this.

But, I agree with @jongiddy that there'd be better ways to accomplish this.
As this PR won't be merged, I suggest, also for the sake of discussion, to create a new PR that adds a new method so making this improvement won't be a breaking change, if there is interest.

Thanks for your understanding.