Implement $bits(type) to get the size of a type

In the process, I implemented a means to get at
previously elaborated types from the pform type
pointer.

PFunction.cc

@@ -65,7 +65,7 @@ void PFunction::push_statement_front(Statement*stmt)
       blk->push_statement_front(stmt);
 }
 
-void PFunction::set_return(const data_type_t*t)
+void PFunction::set_return(data_type_t*t)
 {
       return_type_ = t;
 }