Move all examples to a top-level directory (#1286)

* Move all examples to a top-level directory

This commit moves all API examples (Rust and C) to a top-level
`examples` directory. This is intended to make it more discoverable and
conventional as to where examples are located. Additionally all examples
are now available in both Rust and C to see how to execute the example
in the language you're familiar with. The intention is that as more
languages are supported we'd add more languages as examples here too.

Each example is also accompanied by either a `*.wat` file which is
parsed as input, or a Rust project in a `wasm` folder which is compiled
as input.

A simple driver crate was also added to `crates/misc` which executes all
the examples on CI, ensuring the C and Rust examples all execute
successfully.

examples/multi.rs

@@ -0,0 +1,101 @@
+//! This is an example of working with mulit-value modules and dealing with
+//! multi-value functions.
+//!
+//! Note that the `Func::wrap*` interfaces cannot be used to return multiple
+//! values just yet, so we need to use the more dynamic `Func::new` and
+//! `Func::call` methods.
+
+// You can execute this example with `cargo run --example multi`
+
+use anyhow::{format_err, Result};
+use std::rc::Rc;
+use wasmtime::*;
+
+struct Callback;
+
+impl Callable for Callback {
+    fn call(&self, args: &[Val], results: &mut [Val]) -> Result<(), Trap> {
+        println!("Calling back...");
+        println!("> {} {}", args[0].unwrap_i32(), args[1].unwrap_i64());
+
+        results[0] = Val::I64(args[1].unwrap_i64() + 1);
+        results[1] = Val::I32(args[0].unwrap_i32() + 1);
+        Ok(())
+    }
+}
+
+fn main() -> Result<()> {
+    // Configure our `Store`, but be sure to use a `Config` that enables the
+    // wasm multi-value feature since it's not stable yet.
+    println!("Initializing...");
+    let engine = Engine::new(Config::new().wasm_multi_value(true));
+    let store = Store::new(&engine);
+
+    // Compile.
+    println!("Compiling module...");
+    let module = Module::from_file(&store, "examples/multi.wat")?;
+
+    // Create external print functions.
+    println!("Creating callback...");
+    let callback_type = FuncType::new(
+        Box::new([ValType::I32, ValType::I64]),
+        Box::new([ValType::I64, ValType::I32]),
+    );
+    let callback_func = Func::new(&store, callback_type, Rc::new(Callback));
+
+    // Instantiate.
+    println!("Instantiating module...");
+    let instance = Instance::new(&module, &[callback_func.into()])?;
+
+    // Extract exports.
+    println!("Extracting export...");
+    let g = instance
+        .get_export("g")
+        .and_then(|e| e.func())
+        .ok_or(format_err!("failed to find export `g`"))?;
+
+    // Call `$g`.
+    println!("Calling export \"g\"...");
+    let results = g.call(&[Val::I32(1), Val::I64(3)])?;
+
+    println!("Printing result...");
+    println!("> {} {}", results[0].unwrap_i64(), results[1].unwrap_i32());
+
+    assert_eq!(results[0].unwrap_i64(), 4);
+    assert_eq!(results[1].unwrap_i32(), 2);
+
+    // Call `$round_trip_many`.
+    println!("Calling export \"round_trip_many\"...");
+    let round_trip_many = instance
+        .get_export("round_trip_many")
+        .and_then(|e| e.func())
+        .ok_or(format_err!("failed to find export `round_trip_many`"))?;
+    let args = vec![
+        Val::I64(0),
+        Val::I64(1),
+        Val::I64(2),
+        Val::I64(3),
+        Val::I64(4),
+        Val::I64(5),
+        Val::I64(6),
+        Val::I64(7),
+        Val::I64(8),
+        Val::I64(9),
+    ];
+    let results = round_trip_many.call(&args)?;
+
+    println!("Printing result...");
+    print!(">");
+    for r in results.iter() {
+        print!(" {}", r.unwrap_i64());
+    }
+    println!();
+
+    assert_eq!(results.len(), 10);
+    assert!(args
+        .iter()
+        .zip(results.iter())
+        .all(|(a, r)| a.i64() == r.i64()));
+
+    Ok(())
+}