rust-hg-core: move from `ouroboros` to `self_cell` `ouroboros` has a fundamental soundness problem that, while not applicable today, could become applicable given new compiler optimizations.¹ `self_cell` is a crate that accomplishes a lot of the same things that `ouroboros` did while remaining sound (that is, unless a new soundness issue is discovered) by not assuming as much about the memory layout of the program. `self_cell` has been scrutinized heavily in the past few months by very competent people, some from the compiler team and has shown no weaknesses for a while, with a 1.0 stable release coming out a couple months ago. Our internal API is exactly the same, this is just an implementation detail. To reiterate, no actual soundness issue was found with our use of `ouroboros`, but there might be evolutions of `rustc` (or even a future separate compiler) that could generate unsound code. [1] https://github.com/joshua-maros/ouroboros/issues/88