Remove dependency on golang.org/x

[nhooyr]

See #128 (comment)

Would be nice to be truly zero dep.

Right now we depend on httpguts and xerrors.

xerrors can be easily removed now that Go 1.13 is out. Only problem is no frame information anymore which is unfortunate but welp

For httpguts I think it can be easily removed too.

[andersfylling]

Will you be doing v2 release once this closes?

[nhooyr]

I'll release v1.5.1. I don't think we need to release a v2, xerrors and stdlib errors are fully compatible.

[coadler]

The frame information not making it into std is a big bummer.

[andersfylling]

You can actually use xerrors on go1.13 errors?

[nhooyr]

Yes, it's all based on interfaces so should just work.