Add post on ASN1

One of the posts for mirage/mirage#257

There are still things to do but I'm submitting this PR now so there's plenty of time tomorrow morning.

- Pick a better title. You have better imaginations that me on this
- Update the cross-referencing links at the *end* of the post with the title
- Copy that section into the end of the previous three posts

@pqwy and @hannesm may need to help address my comments inline.