sort: migrate from ouroboros to self_cell

[cakebaker]

The author of ouroboros recommends in someguynamedjosh/ouroboros#88 to migrate to self_cell because of a fundamental soundness problem:

TL;DR: Ouroboros has a soundness problem. A fix has been attempted but it was also unsound. Currently the compiler does not actually produce unsound machine code, but it may begin doing so at any point in the future. Migrate code to use self_cell instead.

[cakebaker]

In the meantime, there has been an update:

UPDATE: A fix has been implemented. 0.16.0+ is no longer unsound in the described way, but support for template parameters had to be cut in the process.

I don't know whether we want to keep using ouroboros by updating it (see #4978) or still switch to self_cell (see #4972).

[tertsdiepraam]

It might also be worth checking if we can get rid of the self-references in general. There's nothing fundamentally about sort that would require self-references. However, that would be a much larger change.