Things to make different next time around (slog v3?)

If this is to ever happen, it should happen after log is 1.0 or has settled enough so there's a clear compatibility story here.

• The whole defaulting to disable debug and trace in release build is probably a bad idea.
• The stuff that is behind flags now, should be standard.
• Drain::log to take &SendSyncRefUnwindSafeKV or something, instead of Owned.
• Make Logger a trait so people can extend it.
• https://github.com/slog-rs/slog/issues/169
• Support named format arguments with nicness like {foo:02X} - probably requires turning macros into procedural macros.
• Use serde-erased instead of Serializer? https://github.com/rust-lang-nursery/log/issues/149#issuecomment-369130459
• use failure::Error for error handling?