Generate visitors automatically

[W95Psp]

This PR:

• drops the manually implemented visitors, which are hard to maintain and error-prone;
• adds a OCaml utility binary generate_visitors that derive those visitors reading the ast.ml module;
• adds a dune rule to generate the module ast_visitors using generate_visitors

Note: the CI is failing for Charon, but that's likely because of rustc updates

[franziskuskiefer]

I'm not sure I understand this PR.

You generate the visitor code but then also commit it? What is the intended workflow here?

[W95Psp]

No, I commit small visitors for the types used by the visitor generator 😅 so yes, that's particularly confusing.
The visitor generator is designed for the types in the module Ast, it doesn't solve the general problem of generating visitors, which is more complicated. But we don't need a general solution, whence that small OCaml utility.

That utility defines a small AST for representing types, and I needed visitors for that small AST, which I generated using the tool itself, but since it's that AST is a bit different, I had to do manual edits. That's why I commit that visitors.ml file.
The visitors of the engine itself are now gone and fully generated.

[franziskuskiefer]

Please add comments to all the new code before merging.