Add FFI wrapper crate

.travis.yml

@@ -23,5 +23,14 @@ before_script:
   - 7z x loot_api-0.13.8-0-g47797cc_dev-win64.7z
 
 script:
-  - cargo fmt -- --check
-  - cargo test
+  - cargo fmt --all -- --check
+  - cargo test --all --all-features
+
+  # Need to rebuild the FFI wrapper so that its binary is given a filename
+  # without a hash.
+  - cargo build --manifest-path ffi/Cargo.toml --features ffi-headers
+  - mkdir ffi/build
+  - cd ffi/build
+  - cmake ..
+  - make
+  - make test