Add an option to keep old behavior of union/structs using pointers

[nick4fake]

Hi,

What do you think about adding possibility to keep using pointers? I don't see any relevant tickets or documentation.

I am talking about this change:
ecdb974

Passing structs requires copying, and using generated "*New" methods is quite difficult as getting pointer to structure requires creating a separate variable each time as go doesn't allow getting reference to return result.