tor-basic-utils: remove `flatten`

BREAKING: Removed flatten().

// ----------------------------------------------------------------------

/// Flatten a `Result<Result<T, E>, E>` into a `Result<T, E>`.

///

/// See [`Result::flatten`], which is not available

/// at our current MSRV.

// TODO MSRV 1.89: When our MSRV is at least 1.89,

// remove this function and replace uses with `Result::flatten`.

pub fn flatten<T, E>(x: Result<Result<T, E>, E>) -> Result<T, E> {

    match x {

        Ok(Ok(x)) => Ok(x),

        Err(e) | Ok(Err(e)) => Err(e),

    }

}

// ----------------------------------------------------------------------

/// Asserts that the type of the expression implements the given trait.

///

/// Example: