Removed string allocation for booleans

[vallentin]

Title and code says it all :)

[dtolnay]

An alternative to consider would be changing call_to_string:

    fn call_to_string<T>(&mut self, v: &T) -> Result<()>
    where
        T: std::fmt::Display,
    {
        use std::fmt::Write as _;
        write!(self.output, "{}", v).unwrap();
        Ok(())
    }

That removes string allocation for not just bool but all the other calls too.