Support one hardcoded import

boot/parser.yy

@@ -182,7 +182,7 @@ program:
         }
     | "module" ";" import_part constant_part type_part variable_part procedure_part "end" "."
         {
-            auto tree = new boot::program(boot::make_position(@1));
+            auto tree = new boot::unit(boot::make_position(@1));
 
             std::swap(tree->imports, $3);
             std::swap(tree->constants, $4);