Allow passing unsigned integer to signed type in/out/ref parameter and vice versa

An example that needs this is AtomicInt methods:

namespace AtomicInt
    public int @get (ref int atomic)
    public void @set (ref int atomic, int newval)
    ....

If a uint value is passed to the atomic parameter, the compiler produces an error; error: Argument 1: Cannot convert from `uint' to `int'. According to GLib documentation, however, the atomic parameter could be either int or uint.

atomic : a pointer to a gint or guint