Add generic document type

[bclavie]

Hey, so sorry it took me so long to get around to this PR.

I like the overall idea and I think we should move towards this model!

My three comments are:

• This change should be fully transparent and not break anything -- I think you've made sure of that in the returned Result format, thanks!
• In the same vein, I don't think we should be forcing user to use our Document type, I think it makes more sense for the user to be able to pass just text and it be auto-constructed from it.
• I can add this myself, but I think Document should also come with a "metadata" attribute which is just a dict where the user can pass in whatever!

I might take a stab at this myself later this afternoon, but thanks again for your contribution, and so sorry for the how long it took me to review this1

[bclavie]

Hey, I've made a number of changes. There was a lot of API breaking change, which is something that will never happen with rerankers (I'm intending to keep everything compatible with the release version for as long as humanly possible). The key result of my updates are basically:

• Result objects are now transparent to their Document object
• Users can freely pass in pre-constructed Documents, string+doc_ids+meta_data, or any combination thereof, and the results will be the same.

I think this is ready to merge now, thank you for getting this started!