Miscellaneous updates

[lrazovic]

• Use Rust 2018 Edition, this bumps MSRV to 1.31.0.
• Use the include filed in Cargo.toml to reduce the crate size by 10%.
• Update libc to latest version.
• Update hermit-abi to latest version.
• Use Self instead of structs name in the impls
• Use MaybeUninit instead of mem::zeroed. This removes an unsafe block, but bumps MSRV to 1.36.0.
• Made Cgroup::new() a const function, this bumps MSRV to 1.32.0.

I understand that it's a lot of changes, and especially for those that increase the level, I'm ready to discuss its implementation and in case go back.

I tested this PR on Linux and Windows, using cargo test --all-features --all-targets, miri and clippy.

[vilgotf]

• Update libc to latest version.

This just disallows older libc versions for no reason. The latest libc version is already allowed (libraries don't generally specify max versions in rust, the specific version is decided by applications in Cargo.lock)

[lrazovic]

Thank you so much for the explanation, now I learned something new. I removed the dependency updates from the PR.

[seanmonstar]

Sorry for missing this 🙈

The dependency change is a good catch, we should stay conservative. Also, any changes that increase the MSRV should probably only be done because of a huge improvement. With such a basic, fundamental crate, we don't want to make it so people couldn't upgrade (even if they are using some ancient compiler).