Backend token/struct API for AbstractFFTs

[nHackel]

This PR adds a backend token api as discussed in this AbstractFFTs PR. It would be great if we could keep the main discussion on the general design in the AbstractFFT PR.

Such an API would close #66.

[dannys4]

@nHackel this is great, and I think significantly better than my (unfortunately forgotten) fix #67 . Ping me when the AbstractFFTs PR goes through and I'd be happy to merge this, pending tests passing.

[nHackel]

@dannys4 Here is a version of the backend variant which solely relies on the backend token and thus results in a breaking change. To not spam PRs, I created a PR in my fork for now.

[dannys4]

looks good, seems to work on my machine when I check out your branches. Feel free to just include all changes in one pr and ping me when the AbstractFFTs changes get made.