Keep BoxedGenerator::args alive

Previously we would keep its elements alive but not the actual array itself?
I think this is what was crashing fasta.py, which is now working with
this change.

I couldn't reproduce the presumed bug in a smaller testcase though.

src/runtime/generator.cpp

@@ -252,6 +252,8 @@ extern "C" void generatorGCHandler(GCVisitor* v, Box* b) {
         v->visit(g->arg2);
     if (num_args >= 3)
         v->visit(g->arg3);
+    if (g->args)
+        v->visit(g->args);
     if (num_args > 3)
         v->visitPotentialRange(reinterpret_cast<void* const*>(&g->args->elts[0]),
                                reinterpret_cast<void* const*>(&g->args->elts[num_args - 3]));