Disallow struct layout changes

This seems like not only an odd feature, but a bad/dangerous one.
When the layout has been used for any existing objects, changing it will produce
very difficult-to-diagnose bugs (at least) or potentially memory violations
if the layout is re-retrieved from the class and used for previous pointers.

It was heavily used in the specs to overwrite one and the same class multiple times to change the layout immediatelly before the test.
However this use case is specific to testing and should not be used anyway.

Fixes ffi#734

lib/ffi/struct.rb

@@ -204,7 +204,7 @@ class << self
       #    end
       #  @note Creating a layout from a hash +spec+ is supported only for Ruby 1.9.
       def layout(*spec)
-        #raise RuntimeError, "struct layout already defined for #{self.inspect}" if defined?(@layout)
+        raise RuntimeError, "struct layout already defined for #{self.inspect}" if defined?(@layout)
         return @layout if spec.size == 0
 
         builder = StructLayoutBuilder.new