Rust backend

[lukaszcz]

• 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.