pub fn to_toml_string(&self, options: &HakariOutputOptions) -> Result<String, TomlOutError> {
        let mut out = String::new();
        self.write_toml(options, &mut out)?;
        Ok(out)
    }
A convenience method around `write_toml` that returns a new string with `Cargo.toml` lines.

The returned string is guaranteed to be valid TOML, and can be provided to
a [`HakariCargoToml`](crate::HakariCargoToml) obtained from [`read_toml`](Self::read_toml).
tools/hakari/src/hakari.rs