add rust-toolchain.toml file

[roman-kashitsyn]

This change introduces a rust-toolchain file that instructs rustup to
use the right version of Rust tooling for the project. See
https://rust-lang.github.io/rustup/overrides.html for more details.

[nomeata]

With this in place, we can probably simplify installing rust in the CI scripts, and deduplicate the information which version to use

[nomeata]

So is there an equivalent to

          rustup update ${{ matrix.rust }} --no-self-update
          rustup default ${{ matrix.rust }}
          rustup target add wasm32-unknown-unknown

that reads the version and target list from this file?

[nomeata]

No, currently not supported, it seems: rust-lang/rustup#2686

[roman-kashitsyn]

With this in place, we can probably simplify installing rust in the CI scripts, and deduplicate the information which version to use

Would be nice, but it's indeed not yet supported. This file mostly useful for local development ATM. It instructs rustup to pick the right version and install the targets automatically. The intention is to reduce friction when onboarding new people or upgrading the compiler.