Switch to go modules and initialize v8 of the project

[gurudesu]

This sets up go modules for this repository and changes the base version to v8.

Additionally, this removes the checked in vendor folder of the project.

This resolves #169

Changes to imports

Imports are now updated to github.com/twitchtv/twirp/v8 for users of v8.0.0 and above.

Other than that, no major changes for even users of dep as long as they are using a version of go > LATEST - 3.

Future major revisions will need to go into the ./v9 folder of the repository if there comes a need for another major revision.

Disclaimer

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[gurudesu]

Opened this to start a conversation around our go modules approach.

Looks fairly heavy handed in terms of folks providing libraries that wrap twirp. They will have to update their libraries every major version release which is not ideal. Exploring a few things so folks won't need to.