feat!: zip 3.0: Adjust flate2-related features for 3.0

[joshtriplett]

Move deflate-zlib to deflate-flate2-zlib to make it clear it's a
flate2 backend, and add deflate-flate2-zlib-rs to use the zlib-rs
backend. Make the latter the default.

Bump the required flate2 version to one that works with zlib-rs 0.5.0,
which removes all compatibility considerations by no longer exporting
any C symbols that would conflict with another zlib.

Remove deflate-zlib-ng, largely superseded by
deflate-flate2-zlib-rs.

Remove no-op backend-specific code from an example.

Add documentation for the flate2-related feature flags.

[Pr0methean]

Thanks! This looks good. Unfortunately, our meagre allotment of 20 GitHub Actions runners has to cope with an expanding matrix of architectures and feature-flag configurations that now includes 69 jobs for each pull request, and several PRs are already in the queue. So it may take a day or two to merge this.

[Pr0methean]

I'll check through and close any PRs that this one clearly renders obsolete.

[joshtriplett]

@Pr0methean I'll fix the CI failures on this PR.

[Pr0methean]

@joshtriplett Looks like the Miri tests need their feature configurations updated in https://github.com/zip-rs/zip2/blob/master/.github/workflows/ci.yaml#L50.

[joshtriplett]

Unfortunately, our meagre allotment of 20 GitHub Actions runners has to cope with an expanding matrix of architectures and feature-flag configurations that now includes 69 jobs for each pull request

Potential improvement for the future: I think it might be reasonable to reduce the matrix somewhat, to test features independently of targets. All targets should test --all-features to make sure that every feature compiles on every target, but other parts of the features matrix need only get tested on one target.

Also, caching would speed things up substantially, for the non-nightly jobs. https://github.com/Swatinem/rust-cache works quite well.

[joshtriplett]

Rebasing atop 5542e5b to avoid stopping other CI jobs if semver-checks fails (which it will for this change).

[joshtriplett]

(Also, I think 5542e5b didn't go far enough, because it only removed the dep for one of the four fuzz jobs.)

[Pr0methean]

Fixed in 7cd9f0f. I look forward to the day when our CI workflow is stable (in the sense of not needing any changes) for long enough that we can factor out a job template or a matrix for the 4 fuzz jobs.

[Pr0methean]

@joshtriplett The doctests for finish_into_readable and merge_archive are failing. I've fixed their feature-flag configurations in master, but could you please fix them if they still fail?