env: Allow zero-allocation LogBuilder::format?

[tomjakubowski]

The the "format function" passed to LogBuilder::format must implement Fn(&LogBuilder) -> String. That is, it must allocate a new string on the heap for every message logged. The string is discarded immediately after env_logger::Logger writes to stderr.

Would you consider, instead, making the signature Fn(&LogBuilder, &mut std::io::Write) -> io::Result? That way, customized env loggers could write directly to stderr without creating a String.

[KodrAus]

Thanks for the suggestion @tomjakubowski! We did end up doing this in rust-cli/env_logger#19