Implement a Rust FFI for embedding bytecode generation in C++

Summary:
Bytecode generation entry point `compile_ffi::compile_from_text_cpp_ffi(...)` and a driver to exercise it:  `hh_single_compile_cpp`.

What we get here is a C FFI for generating HHAS from source text. We can tweak the interface as we need to going forward but this will do for a first version and I think has roughly feature parity with the OCaml FFI:
- `compile_ffi.rs` defines the function;
- `compile_ffi.h` defines the C interface;
- `hh_single_compile_cpp.cpp` provides a C++ CLI that calls the function.

Reviewed By: shiqicao

Differential Revision: D25979967

fbshipit-source-id: 4b46f9af23c61150dda6c33f9fa14e2c455c54c2

eden/scm/lib/hgcommits/Cargo.toml

@@ -6,7 +6,7 @@ version = "0.1.0"
 include = ["src/**/*.rs"]
 
 [dependencies]
-dag = { path = "../dag", features = ["for-tests"] }
+dag = { path = "../dag", features = ["for-tests", "indexedlog-backend"] }
 edenapi = { path = "../edenapi" }
 gitdag = { path = "../dag/gitdag" }
 metalog = { path = "../metalog" }