Update Lean to current nightly

[ineol]

There is one lemma that I couldn't easily prove, because the definition of fixed width integers changed

[gleachkr]

Thanks, and sorry for the slow reply. At the moment, I'd like to make sure not to diverge from https://github.com/leanprover/LNSym (unless someone over there tells me it's safe to do so), but if you notice that they've advanced their lean version, please let me know and I'll bump things on this end.