split create_binary into two phases

Which gives instructions a chance to check everything
and in Arms case check the constant loads/ instruction adding
So that during assembly no more change happens (and we don't have to reassemble)

lib/util/dev_null.rb

@@ -0,0 +1,7 @@
+module Util
+  # A class that does not write, or swallows all incoming data
+  # Used in Arm to do a dry run (and in testing, where it was born)
+  class DevNull
+    def write_unsigned_int_32( _ );end
+  end
+end