Function write_to 

pub fn write_to(
    hosts: &[String],
    days: u32,
    cert_path: &str,
    key_path: &str,
) -> Result<SelfSigned>

Generate a certificate for hosts and write it to cert_path/key_path.

Creates parent directories as needed. The key is written 0600 on Unix — a private key that lands world-readable is a worse outcome than the missing certificate we set out to fix.