Make the flate2 dependency optional

[eminence]

CC #156

[pymongo]

I think && cfg!(feature = "flate2") is useless

[pymongo]

if cfg!(feature = "flate2") == false, block would not execute

if cfg!(feature = "flate2") == true, block would execute flate2::read::GzDecoder

so the cfg(not(...flate2 block may never execute

[pymongo]

if cfg!(feature = "flate2") == false, block would not execute

if cfg!(feature = "flate2") == true, block would execute flate2::read::GzDecoder

so the cfg(not(...flate2 block may never execute

[eminence]

Yes, the #[cfg(not(feature = "flate2"))] block is unreachable, but is needed to typecheck

[pymongo]

Other prosible conditional compile logic:

1. if feature = "flate2" disable, not compile the pub fn kernel_config function, and all unittest relative it add cfg(feature="flate2") attr
2. like procfs::boot_times(), in chrono feature return Datetime while not chrono feature return i64, I think kernel_config not flate2 feature can return bytes, and flate2 feature return HashMap

[pymongo]

This PR looks good to me, I can remove the flate2 dependency and speed up my project compile

[eminence]

This is a breaking change, since anyone using procfs with default-features = false will see an incompatible change in behavior for the kernel_config function. This is unfortunate, but I'm going to live with it. We'll put a big notice in the release notes about this breaking change. I'll also looked many crates that depend on procfs and use default-features = false, and none use the kernel_config function.