There should be a way to push down a virtual function defined in ddl

If you create a function that is defined in ddl, it currently requires that a translator be extended to add a pushdown function and handling. It would be better if there were a way to do this entirely in ddl. This could be either/or:

1. when the function is defined by an expression rather than a more complex procedure statement, we should check to see if that can be inlined.
2. provide a way to match source functions.