Official Support for 3rd-Party Backends

This release marks the finalization of pluggable backends feature.

This release should be the bare minimum that should be used to create custom backends. Further releases will try to keep backend support stable but may not entirely reliable. However, the backends created using this release are bound to always be supported unless further releases confirm otherwise.

This release is not for consumer use and contains no pre-built backends. Dynamic building mechanism is included but only for developers (or whoever is willing to modify Cargo.toml and call a build manually).