Add external logs (loggable objects) for use with the slog ecosystem.

Structured, contextual, extensible, composable logging for Rust

Json formatter for slog-rs

Syslog drain for slog-rs

Asynchronous drain for slog-rs v2

Unix terminal drain and formatter for slog-rs

Key values based filter Drain for slog-rs

Standard Rust log crate adapter to slog-rs

Port of `env_logger` as a `slog-rs` drain.