RCR 219: add Kernel.warn!() for warnings with backtraces

There should be a Kernel method warn!(), a variation of Kernel.warn() which would print out a backtrace for debugging purposes.

It is often useful to know the exact point in your script from which a warning was issued. Currently, there is no standard method or framework for doing this.

There should be a method added to the Kernel module, which prints not only the warning, but the location in the script that caused the warning.

Usually, the bang at the end of a method name indicates that the object will be modified. You can extend that to imply danger, or something done more throughly. Kernel.warn!() implies both of these, as warnings given in this way would be for more dangerous conditions (though not warranting an exception) and would more thoroughly report the error (by including a backtrace).

I imagine it would probably be written into the Kernel module, but here's an example of how it might be done in standard Ruby.

module Kernel
    def warn!(warning)
        warn( "Warning: #{warning}\n"+
            caller.map{|a|' '*8+"from "+a}.join("\n")
        )
    end
end