add assert and [[nodiscard]] for spinlock

[poor-circle]

Why

1. Close async_simple::coro::Mutex should contain <cassert> head file #391
2. We found some mistake use of spinlock in user issue

What is changing

1. add cassert header for Mutex.h
2. add nodiscard attribute for SpinLock::coScopedLock()
3. add assert for SpinLock::unlock()

Example

[ChuanqiXu9]

Please take a look at the cmake failure