This PR adds codegen from rustc to track the file and line number of
calls to `log_err()`. I haven't noticed much longer compile times on my
machine, and looking at the
[implementation](https://rustc-dev-guide.rust-lang.org/backend/implicit-caller-location.html)
it essentially adds an extra argument and secret reference pass.
However, this will show a lot more data in our logs on user machines.
Requesting review from @ForLoveOfCats, who usually knows a bunch about
this kind of thing :)