Remove xerrors dependency and use stdlib.

[googollee]

No description provided.

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[dsnet]

I apologize, but we can't accept this change. See #225 (comment)

[googollee]

I understand that it needs to support Go 1.8. As go mod has version control, could we raise the version number to drop the dependency? If users want to use it under Go 1.14, they could pin the version 0.5.x.

[dsnet]

See #225 (comment)

[googollee]

Thanks for clarifying.