allow get_slot with two registers

before was register and number, but for runtime that doesn’t work

lib/register/instructions/get_slot.rb

@@ -23,8 +23,8 @@ module Register
       @array = array
       @index = index
       @register = register
-      raise "index 0 " if index == 0 
-      raise "not integer #{index}" unless index.is_a? Numeric
+      raise "index 0 " if index == 0
+      raise "Not integer or reg #{index}" unless index.is_a?(Numeric) or RegisterValue.look_like_reg(index)
       raise "Not register #{register}" unless RegisterValue.look_like_reg(register)
       raise "Not register #{array}" unless RegisterValue.look_like_reg(array)
     end