Clircle 0.6.0

This release changes how the Identifier type is defined. Instead of aliasing the platform implementation, a new type is created that (privately) wraps the specific implementation, so that the internals can be changed without breaking the public interface. In that spirit, the re-exports of libc and winapi have been removed.

Additionally, to aid packaging, the nix dev-dependency is updated to the latest version. This required a bump of the MSRV to Rust 1.69.