Skeleton code for decls-to-facts conversion
Summary:
Add an FFI that transforms Decls into Facts
```
unsafe fn hackc_decls_to_facts_cpp_ffi<'a>(
decls: &Decls<'a>,
source_text: &CxxString,
) -> FactsResult;
```
Also add conversation traits to enable the flow "shared Decls" --> "rust Decls" --> "rust Facts" --> "shared Facts", with main logic to be implemented in "rust Decls" --> "rust Facts" stage.
Reviewed By: shayne-fletcher
Differential Revision:
D31658146
fbshipit-source-id:
9646b7383ed40f8b80f164a0e7e5d738c88de983