[rustc toolchain]: update to latest, avoid known rust issues

[niklasad1]

Update rustc toolchain to ensure that we are not affected by https://blog.rust-lang.org/2020/02/27/Rust-1.41.1.html. We are using rustc 1.42.0-nightly (3291ae339 2020-01-15) seems to have those issues (please correct me if I'm wrong)

Related Rust PRs:

• [stable] Cherry-pick LLVM fix for 1.41.1 rust-lang/rust#69444
• Fix MIR typeck soundness holes rust-lang/rust#69145

[bradjc]

I used this as an opportunity to continue to test my update_rust_version script, and this jump is the first time that I can remember where Tock actually does not compile (for hail) with a new rust version. Also, it does seem that we are "exploiting" the static bug currently, but in practice I don't think it has had any effect.