Write a GitHub action that executes the same test for current development and lowest supported Rust toolchain

[mulkieran]

The first is automatically obtained from rust-toolchain.toml, the second can be read from the Cargo.toml rust-version field. At present, the value is read from Cargo.toml using a Python library, toml-cli to avoid bootstrapping from Rust to install Rust. In future might be easier to install the toolchain specified in rust-toolchain.toml, install cargo-get using that toolchain, run the tests using that toolchain, and then obtain the lowest supported toolchain using cargo-get, set the override using that toolchain, and then run the tests again.

[mulkieran]

Better just to make it one that switches from current development to lowest supported as an override in the repo to test, for better modularity.