Add autosubst-ocaml to the Coq Platform

[palmskog]

autosubst-ocaml is an OCaml reimplementation of the (seemingly unmaintained) Autosubst 2 code generator. Autosubst 2 in turn is based on previous work on Autosubst, but is a code generator rather than a Coq library.

To improve the Platform's support for programming language metatheory, I propose that autosubst-ocaml is included. Technically, autosubst-ocaml interfaces with Coq's API and is part of Coq's CI.

@yforster, can you confirm here that releases/tags can be done of autosubst-ocaml for every major Coq/Rocq version? The work should be minimal for this since updates will come via Coq's CI.

[palmskog]

As discussed on Zulip, Autosubst1 could in the future be reimplemented as an interface at Coq level to the code in autosubst-ocaml. However, until that happens, it should make sense to include both Autosubst1 (Coq library) and autosubst-ocaml (codegen tool).

[yforster]

I can confirm that we can release / tag for major versions