Engine: generate AST destructors

[W95Psp]

This is basically implemented very partially by hand in Ast_utils.Make(F).Expect.
We want destructors for each constructor of expr', pat', item' that work directly on expr, pat and item.

From those basic generated destructors, we'll probably want more precise destructors, e.g. the existing concrete_app1 which expects a function application with one argument and whose function is an immediate concrete ident.
Such destructors will be simple to define via the generated ones.