Attributes: introduce them in engine's AST

[W95Psp]

Currently, attributes are translated from the rust compiler to our THIR' JSON file, but we drop them in the module Import_thir in the engine.

• [x] have attributes on item
• [ ] have attributes on expression
• [ ] have attributes on other specific places (e.g. arms)

[W95Psp]

Re-opening this issue, PR #106 only added support in the frontend exporter, not in the engine.