Function render_to_string 

pub fn render_to_string<F>(estimated_size: usize, render: F) -> String
where
    F: FnOnce(&mut StringBuf<'_>) -> usize,

Convenience: render a readout into a freshly allocated String. Used by the in-process tests and the crate::diag! bridge in Push 1; later pushes will route directly through a borrowed buffer instead.