fn parse_str<F>(&self, key: &str, default: F) -> WeldResult<F>
    where
        F: FromStr,
    {
        self.parse_map(key, default, &mut |v| Ok(v))
    }
Retrieves the value mapped to `key` and parse it to type `F`. Returns `default` if key does
not exist.
weld/src/conf/mod.rs