Add an initial wasi-nn implementation for Wasmtime (#2208)

* Add an initial wasi-nn implementation for Wasmtime

This change adds a crate, `wasmtime-wasi-nn`, that uses `wiggle` to expose the current state of the wasi-nn API and `openvino` to implement the exposed functions. It includes an end-to-end test demonstrating how to do classification using wasi-nn:
 - `crates/wasi-nn/tests/classification-example` contains Rust code that is compiled to the `wasm32-wasi` target and run with a Wasmtime embedding that exposes the wasi-nn calls
 - the example uses Rust bindings for wasi-nn contained in `crates/wasi-nn/tests/wasi-nn-rust-bindings`; this crate contains code generated by `witx-bindgen` and eventually should be its own standalone crate

* Test wasi-nn as a CI step

This change adds:
 - a GitHub action for installing OpenVINO
 - a script, `ci/run-wasi-nn-example.sh`, to run the classification example

.github/workflows/main.yml

@@ -51,6 +51,7 @@ jobs:
     runs-on: ubuntu-latest
     env:
       RUSTDOCFLAGS: -Dbroken_intra_doc_links
+      OPENVINO_SKIP_LINKING: 1
     steps:
     - uses: actions/checkout@v2
       with:
@@ -252,6 +253,7 @@ jobs:
             --all \
             --exclude lightbeam \
             --exclude wasmtime-lightbeam \
+            --exclude wasmtime-wasi-nn \
             --exclude peepmatic \
             --exclude peepmatic-automata \
             --exclude peepmatic-fuzzing \
@@ -304,6 +306,23 @@ jobs:
         CARGO_VERSION: "+nightly"
         RUST_BACKTRACE: 1
 
+  # Build and test the wasi-nn module.
+  test_wasi_nn:
+    name: Test wasi-nn module
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          submodules: true
+      - uses: ./.github/actions/install-rust
+        with:
+          toolchain: nightly-2020-08-25
+      - run: rustup target add wasm32-wasi
+      - uses: ./.github/actions/install-openvino
+      - run: ./ci/run-wasi-nn-example.sh
+        env:
+          RUST_BACKTRACE: 1
+
   # Verify that cranelift's code generation is deterministic
   meta_determinist_check:
     name: Meta deterministic check
@@ -411,6 +430,7 @@ jobs:
             --all \
             --exclude lightbeam \
             --exclude wasmtime-lightbeam \
+            --exclude wasmtime-wasi-nn \
             --exclude peepmatic \
             --exclude peepmatic-automata \
             --exclude peepmatic-fuzzing \