Make flate2 a feature flag

[fintelia]

Right now this crate ships with both fdeflate and flate2 as dependencies. There's a lot of duplicated code between them including two full decompressors and a chunk of duplicated logic on the encode path. If fdeflate gains a general purpose compressor, it would be nice for us to remove the duplication by default.

This PR moves the user-facing guarantee that flate2 is used as the compression backend behind a non-default feature flag. Internally, flate2 is still used for all compression levels other than NoCompression and FdeflateUltraFast.

[Shnatsel]

I agree with the motivation. However, I find this particular implementation rather confusing from the API user perspective. For the most common use case of opting in to zlib-rs for faster compression than miniz_oxide, the steps to enable that really aren't clear.

In the spirit of making easy things easy and hard things possible, I think we should just outright rename Flate2 variant to Level and add zlib-rs toplevel feature that enables zlib-rs for encoding. All the other uses for messing with flate2 are sufficiently niche that they're not worth complicating the API surface for everyone.

[Shnatsel]

I'll make a PR for the option I suggested and we can see if that's any better