Rune now support null safety

[waywardgeek]

Variables and functions can now be declared non-null. E.g.

// Copy a rectangle.  Input and returned value cannot be null.
func copyRect(rect: Rectangle)  -> Rectangle {
    return Rectangle(rect.left, rect.bottom, rect.right, rect.top)
}

// If rect is null, return null, otherwise return a copy of rect.
func copyRectIfNotNull(rect: Rectangle?) -> Rectangle? {
    if isnull(rect) {
        return rect
    }
    return copyRect(notnull(rect)
}

This feature did not catch any bugs in the Rune code we have so far, so it does isn't a huge win, but Rune is meant to be safe, so we're going with null safety.