Using as cast with callables

"kbarros" identified this issue.

To cast a method to a specific callable type, right now we can only use cast() or implicit casts. As casts always fail. Example below. I attached a patch that I hope is the right way to allow as casts to work.

callable OutputHandler(message as string)

def printit(message as string):
System.Console.WriteLine(message)

//works
handler as OutputHandler = printit
handler("Testing...")

//doesn't work, null ref exception
handlerb = printit as OutputHandler
handlerb("Testing B...")