Tracking Status of Grabbing Function Name

[theGeekPirate]

Thank-you very much for this crate, it's fantastic! 🥇

I'm not sure how you'd like to handle the inability to grab the function name currently, but I figured a tracking issue would suffice. Feel free to close this if you disagree!

rust-lang/rfcs#1743

[tmccombs]

Sound good to me. You may want to open a tracking issue in slog-rs/slog as well, since it's probably desirable for the slog macros to capture the function name along with the other source information (line, file, module_path, etc).