Add dependencies to dune-project

[raoxiaojia]

It might be a good idea to add the dependencies to spectec/dune-project. I've done some experiments on building it:

• The project seems to only build with OCaml 5.x (no longer builds on 4.14.0);
• The only dependencies (for the main spectec, without mechanisation backends) seem to be mdx+menhir+zarith.

[rossberg]

Sounds good to me, though modulo yesterday's accidental breakage (fixed now), spectec should still build fine on 4.14.