* Adds a RISC0 backend which generates Rust code that can be compiled
with the official RISC0 toolchain.
* The RISC0 backend is a wrapper around the Rust backend.
* Adds the `risc0-rust` to the `compile` CLI command, which creates a
directory containing host and guest Rust sources for the RISC0 zkVM. The
generated code can be compiled/run using `cargo` from inside the created
directory (requires having RISC0 installed:
https://dev.risczero.com/api/zkvm/install).
* Implements code generation through Rust.
* CLI: adds two `dev` compilation targets:
1. `rust` for generating Rust code
2. `native-rust` for generating a native executable via Rust
* Adds end-to-end tests for compilation from Juvix to native executable
via Rust.
* A target for RISC0 needs to be added in a separate PR building on this
one.
