Hax hangs on a recursive trait with associated type

[Nadrieril]

pub trait Trait1 {
    type T: Trait1;
}

pub trait Trait2: Trait1 {
    type U;
}

Running hax json on this code causes it to hang forever. Removing type U; fixes the issue.

[W95Psp]

This issue stems from the path_to function doing a depth-first search.
Doing a breadth-first search instead solves this issue, will push a PR soon.

Note that switching to a breadth-first search is not solving issue #686.

Pushed #730