Testing infrastructure for rust

[belovdv]

Running rust tests after #3 has following problems:

• It requires python installation with a non-standard library.
• It pollutes the repository.

[belovdv]

Suggestion:

• Language-specific integration tests should only check roundtrip, probably using tempdir.
  • This test allows overwriting paths through environment variables.
• Since we use an under-specified file format, there should be script for cross-language integration testing.
  • It should use an integration test (or separate executable) to transfer data across languages.
  • It can use utilities like yaml_canonicalize.py if necessary.
• CI invokes unit/integration testing per-language, and this script.