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

crates/wasi-nn/examples/wasi-nn-rust-bindings/src/error.rs

@@ -0,0 +1,76 @@
+use super::NnErrno;
+use core::fmt;
+use core::num::NonZeroU16;
+
+/// A raw error returned by wasi-nn APIs, internally containing a 16-bit error
+/// code.
+#[derive(Copy, Clone, PartialEq, Eq, Ord, PartialOrd)]
+pub struct Error {
+    code: NonZeroU16,
+}
+
+impl Error {
+    /// Constructs a new error from a raw error code, returning `None` if the
+    /// error code is zero (which means success).
+    pub fn from_raw_error(error: NnErrno) -> Option<Error> {
+        Some(Error {
+            code: NonZeroU16::new(error)?,
+        })
+    }
+
+    /// Returns the raw error code that this error represents.
+    pub fn raw_error(&self) -> u16 {
+        self.code.get()
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{} (error {})", strerror(self.code.get()), self.code)?;
+        Ok(())
+    }
+}
+
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Error")
+            .field("code", &self.code)
+            .field("message", &strerror(self.code.get()))
+            .finish()
+    }
+}
+
+/// This should be generated automatically by witx-bindgen but is not yet for enums other than
+/// `Errno` (this API uses `NnErrno` to avoid naming conflicts). TODO: https://github.com/bytecodealliance/wasi/issues/52.
+fn strerror(code: u16) -> &'static str {
+    match code {
+        super::NN_ERRNO_SUCCESS => "No error occurred.",
+        super::NN_ERRNO_INVALID_ARGUMENT => "Caller module passed an invalid argument.",
+        super::NN_ERRNO_MISSING_MEMORY => "Caller module is missing a memory export.",
+        super::NN_ERRNO_BUSY => "Device or resource busy.",
+        _ => "Unknown error.",
+    }
+}
+
+#[cfg(feature = "std")]
+extern crate std;
+#[cfg(feature = "std")]
+impl std::error::Error for Error {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn error_from_success_code() {
+        assert_eq!(None, Error::from_raw_error(0));
+    }
+
+    #[test]
+    fn error_from_invalid_argument_code() {
+        assert_eq!(
+            "Caller module passed an invalid argument. (error 1)",
+            Error::from_raw_error(1).unwrap().to_string()
+        );
+    }
+}