fix(engine) Use ocamlgraph fork to fix missing rec bug.

[maximebuyse]

Fixes #1158

This changes the version of ocamlgraph we use to avoid backtracking/ocamlgraph#149. The ocamlgraph is implemented in this fork/branch: https://github.com/maximebuyse/ocamlgraph/tree/fix-stable-topological-sort

In the case of functions that are mutually recursive with an impl method, the examples still don't work. Reproducer:

pub trait A{
    fn f(&self);
}
impl A for i32 {

    fn f(&self){
        g(*self)
    }
}
pub fn g(x: i32) {x.f()}

Open this code snippet in the playground

But this is documented in #616

[W95Psp]

LGTM!