pub fn write_comment(&self, mut out: impl fmt::Write) -> Result<(), TomlOutError> {
        // Begin with a comment.
        let summary = self.to_string().map_err(|err| TomlOutError::Toml {
            context: "while serializing HakariBuilderSummary as comment".into(),
            err,
        })?;
        for line in summary.lines() {
            if line.is_empty() {
                writeln!(out, "#")?;
            } else {
                writeln!(out, "# {}", line)?;
            }
        }
        Ok(())
    }
Serializes this summary to a TOML string, and adds `#` comment markers to the beginning of
each line.

Returns an error if writing out the TOML was unsuccessful.
tools/hakari/src/summaries.rs