Add a rust-toolchain.toml

[kevinheavey]

This example always breaks for people because they try to run the tests with an unsupported Rust version. If there was a rust-toolchain.toml file in src/program-rust this wouldn't be a problem

[kevinheavey]

Oh and I suppose the test:program-rust command in the package.json should also specify the Rust version (not sure if it can be made to read the rust-toolchain.toml)