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/actions/install-openvino/README.md

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+# install-openvino
+
+A GitHub action to install OpenVINO from a package repository. This is only necessary for `wasi-nn` support but there
+are enough steps here to package the functionality separately and avoid cluttering the CI.
+
+Future improvements:
+ - make this installer work for different OS/distributions (e.g. https://docs.openvinotoolkit.org/latest/openvino_docs_install_guides_installing_openvino_windows.html)
+ - it would be nice to output the install directory (i.e. `/opt/intel/openvino`)

.github/actions/install-openvino/action.yml

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+name: 'Install OpenVINO'
+description: 'Install OpenVINO binaries from a package repository; this is significantly faster than building from source'
+
+inputs:
+  version:
+    description: 'The release version of OpenVINO to install'
+    required: false
+    default: '2020.4.287'
+
+runs:
+  using: composite
+  steps:
+    - run: ${{ github.action_path }}/install.sh ${{ inputs.version }}
+      shell: bash

.github/actions/install-openvino/install.sh

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+#!/bin/bash
+
+set -e
+
+# Retrieve OpenVINO checksum.
+wget https://apt.repos.intel.com/openvino/2020/GPG-PUB-KEY-INTEL-OPENVINO-2020
+echo '5f5cff8a2d26ba7de91942bd0540fa4d  GPG-PUB-KEY-INTEL-OPENVINO-2020' > CHECKSUM
+md5sum --check CHECKSUM
+
+# Add OpenVINO repository (deb).
+sudo apt-key add GPG-PUB-KEY-INTEL-OPENVINO-2020
+echo "deb https://apt.repos.intel.com/openvino/2020 all main" | sudo tee /etc/apt/sources.list.d/intel-openvino-2020.list
+sudo apt update
+
+# Install OpenVINO package.
+sudo apt install -y intel-openvino-runtime-ubuntu18-$1