Monitor#mon_try_enter

static VALUE
monitor_try_enter(VALUE monitor)
{
    struct rb_monitor *mc = monitor_ptr(monitor);

    if (!mc_owner_p(mc)) {
        if (!rb_mutex_trylock(mc->mutex)) {
            return Qfalse;
        }
        RB_OBJ_WRITE(monitor, &mc->owner, rb_fiber_current());
        mc->count = 0;
    }
    mc->count += 1;
    return Qtrue;
}