allow reverse syntax for >> (ie <<)

slot >> reg makes sense, being a slot_to_reg
but . . .
consistently use of << (as meaning assignment , =) also makes sense
allow both and let time tell which makes more sense

lib/risc/risc_value.rb

@@ -68,6 +68,8 @@ module Risc
         ins = Risc.load_constant("#{load.class} to #{self.type}" , load , self)
       when RiscValue
         ins = Risc.transfer("#{load.type} to #{self.type}" , load , self)
+      when RValue
+        load >> self
       else
         raise "not implemented"
       end