Decode context.Canceled, context.DeadlineExceeded etc as a reference to the stdlib instance

Currently, instances of the stdlib errors trasnferred over the network are decoded into an object of the same type/value, but not the same instance. So reference comparisons with `==` with the stdlib instance fail.

This matters for e.g. `context.Context.Err()` which describes in its API doc that it returns the standrd errors _unwrapped_ so it's legitimate for a caller to use `==`.

We could address that by adding a special case in the leaf decoder, so that if a leaf is one of the known stdlib errors, we return the stdlib instance instead.

首先阅读 issue 中提到的 leaf decoder，并跟踪通过网络传输的标准库错误是如何被重建的。检查 context.Canceled 和 context.DeadlineExceeded 的表示方式，然后验证解码返回的是标准库实例，从而使与 == 的比较能够成功。

由索引模型根据 Issue 内容生成。