6.4.0

This release contains two feature requests from @lopopolo. We no longer rely on the
older lazy_static crate for shared mutex initialisation, and the crates have been bumped
to 2018.

What's Changed

• Replace lazy_static with once_cell by @lopopolo in #169
• Upgrade to 2018 edition by @lopopolo in #170

Full Changelog: v6.3.2...v6.4.0