xo-object: generative GC utest + reinstate coverage build

xo-object/src/object/List.cpp

@@ -16,6 +16,11 @@ namespace xo {
         gp<List>
         List::nil = new List(nullptr, nullptr);
 
+        gp<List>
+        List::from(gp<Object> x) {
+            return dynamic_cast<List *>(x.ptr());
+        }
+
         gp<List>
         List::cons(gp<Object> car, gp<List> cdr) {
             return new (MMPtr(mm)) List(car, cdr);