When marshalling a closure, try to detach funcenvs

If possible, this will reduce the need to marshal fibers
in many cases. Also add this logic to the GC so holding a closure
that originally came from a fiber that crashed does not cause that fiber
to hang around forever.

src/core/marsh.c

@@ -184,6 +184,7 @@ static void marshal_one_env(MarshalState *st, JanetFuncEnv *env, int flags) {
         }
     }
     janet_v_push(st->seen_envs, env);
+    janet_env_maybe_detach(env);
     pushint(st, env->offset);
     pushint(st, env->length);
     if (env->offset) {