Add Autosubst 2 implemented in OCaml for Coq 8.20

[TheoWinterhalter]

No description provided.

[palmskog]

@TheoWinterhalter your PR title says "Autosubst 2", but the package description says just "Autosubst", which is likely to lead to confusion with Autosubst 1, which has a different interaction mode and interface. How about changing the package description to make this a bit clearer (at least saying "Autosubst 2")?

[TheoWinterhalter]

Then this would be inconsistent with the 8.19 version. Should I update both?
I feel like this is not my prerogative since I'm not even a maintainer. Do I get your approval @yforster?

[yforster]

Absolutely feel free to update both! (I am, however, unsure what the point of these texts is. From my perspective a link to the GitHub repository could also be good enough)

[palmskog]

@yforster the package description shows up on the website, when doing opam search, and hopefully in Coq Platform package collection. The goal here is just to lower the confusion around "Autosubst" for users.

[TheoWinterhalter]

Alright. I added the 2 in the two 1.1 packages.