Explainer document

[dhil]

This PR includes a draft of the explainer document. It also contains some trivial changes to the reference interpreter and formal spec, as it renames event to tag.

[rossberg]

I think if you add suggestions to batch, you avoid spamming subscribers with dozens of individual update mails. :)

[slindley]

Yep. I wanted to batch, but on the default page I get for the pull request there's a very helpful button 'Add suggestion to batch' which is greyed out with a pop up that says 'Batching must be done from the files tab'. Eventually I did give up and moved to the file interface which is also pretty rubbish but at least allows batching.

I think it would be good to go ahead and add handlers for resume_throw.