This makes the `chrono` and `flate2` dependencies optional, adding a `time` and `compression` feature for each. These features are enabled by default, but can be disabled by using `default-features = false` in the Cargo.toml dependency definition, allowing library users to opt out of these dependencies if they don't need them. This should reduce compile times and binary size. In my testing on a release build I saved 1.3 seconds of compilation time (although this is very CPU dependent) and 66 KiB of binary size by dropping these features from a project that depends on file-rotate.

`cfg` attributes have been added to tests to support testing with features disabled, and I've added additional test runs in the rust.yml GitHub Action to cover these cases. If you want me to add additional cargo check runs for these cases let me know and I'll do that as well. The example and doc tests only work with the default features enabled, but I think that's fine.

I have not bumped the library version in Cargo.toml as I believe that should be left to the maintainer. However, note that a major version bump is needed (e.g. 0.7.6 -> 0.8.0) as this change is breaking for anyone already using `default-features = false` (even though there are currently no features someone may still be doing this for some reason). I've seen other projects get hit by this, and affected library users typically call for a cargo yank.