This fixes the directory target rule problem.

Thanks to Rudi Grinberg!!

We move vendored things to `vendor` and let dune know about it.
We bump serapi dep to the head of the v8.16 branch.

We also adjust build deps a bit better, in particular to avoid Dune
3.4 which is buggy for us.

Beware this will require `git submodule update` on the work tree.

SerAPI 0.15 is not compatible with newer `ppx_sexp_conv`.

zarith stub is now included in js_of_ocaml.
There is a slight change with the way dynlink libs are handled.

Because of some stubs.
Closes #258.

This should nicely support M1-based OSX ; unfortunately, it required
loosening the constraint on `ppx_deriving_yojson` as to make it
co-installable with `ppx_import`, but that's a minor issue.

Note the new option system for OCaml Opam packages, thus selecting a
32bit compiler is done differently now.

We also bump some other libraries as to reduce non-deterministic
developer switches.

Nothing too remarkable, code-wise, but a big change upstream is the
addition of `zarith` as a hard dependency.

TODO:

- update addons

This includes some bug fixes and the upstreaming of Shachar's Str
stub, so we can remove it.

We do require a recent toolchain [except for the compiler which is
stuck at 4.07.1 due to ppx_import]

Fixes #87

We depend on serlib to serialize Coq data structures and remove the
duplication present in jsCoq. This greatly expands the range of Coq's
types available for sending as JSON.

Next commit will add a query to showcase UI grabbing the full AST for
a sentence, which allows pretty cool things UI-wise.

For now, we include serlib as a module, which is a bit cumbersome,
however once we release serapi 0.7.0 we should be able to just depend
on the coq-serapi OPAM package.

Closes #89

There is some extra deprecation that we need to take care of

This may not be the best place to put it, but the build fails on
a clean `opam switch 4.07.1+32bit` without these.

We do some cleanup in examples, addons, and miscellaneous bits.

All the changes were originally done in the Dune PR (#37), but as it
is delayed I've cherry-picked the bits that don't depend on the Dune
build itself.

Should mostly supersede #57.