Set up parsing integration tests with simple rust programs

[jberthold]

To test the MIR extraction and parser infrastructure on more real-world input, a collection of small Rust programs should be added to discover regressions in the processing chain. This also means integrating smir_pretty into mir-semantics, as we don't want to check in the intermediate stable-MIR artefacts which are still subject to changes.

• salvage old rust test programs from textual-mir branch
• integrate smir_pretty into integration test setup
  • This could use either nix (if all repos are nix-ready) or git submodule and make (for now)
• test parsing the programs (successful smir_pretty run and successful parse of the result)
• fix parser errors/deficiencies discovered in the tests