Update rust toolchain to 1.91.1

[zetanumbers]

Decreases rust's incremental compilation roughly by a factor of 3.

Specifically tests' build time with:

touch crates/compiler/parse/src/parser.rs # retrigger incremental compilation with zero changes
cargo build --tests

on i7-13700 decreases from 15.86s down to 5.42s.

Clean build time also decreased from 42.07s to 31.24s.

[Anton-4]

Hi @zetanumbers,
We've been hard at work on the new compiler written in zig (see src folder) so we have not upgraded the old compiler written in rust. I suspect it will take much more changes before you can get all of CI to pass. If you want to make those changes I will review them but we do not see substantial value in a rust upgrade of the old compiler. Sorry to disappoint!

[zetanumbers]

After some testing I can say a large portion (16.05s -> 8.13s) of build time reduction was caused by rust switching to lld by default on x86_64 linux (rust-lang/rust#140525).

[Anton-4]

Oh yeah, we were using lld in some places but not everywhere I think. I would still prefer to avoid making the switch now, the old compiler had several flaky issues and changing things up here could make them worse

[zetanumbers]

You should delete the old compiler code if it's not maintained.

[Anton-4]

The new compiler is still missing a lot of features, so we keep the old one around. We also still fix docs issues and other issues that are really bothering people. I just don't want to risk making the compiler or CI more flaky for a speedup that would mainly benefit compiler developers.