Add example of execution limits using fuel consumption (#2869)

* Add example of execution limits using fuel consumption

* run rustfmt

* Use a more naive WAT implementation

* Catch error and return cleanly

* Add a C example to demonstrate fuel consumption

* Add error handling for add_fuel

examples/fuel.rs

@@ -0,0 +1,33 @@
+//! Example of limiting a WebAssembly function's runtime using "fuel consumption".
+
+// You can execute this example with `cargo run --example fuel`
+
+use anyhow::Result;
+use wasmtime::*;
+
+fn main() -> Result<()> {
+    let mut config = Config::new();
+    config.consume_fuel(true);
+    let engine = Engine::new(&config)?;
+    let store = Store::new(&engine);
+    store.add_fuel(10_000)?;
+    let module = Module::from_file(store.engine(), "examples/fuel.wat")?;
+    let instance = Instance::new(&store, &module, &[])?;
+
+    // Invoke `fibonacci` export with higher and higher numbers until we exhaust our fuel.
+    let fibonacci = instance.get_typed_func::<i32, i32>("fibonacci")?;
+    for n in 1.. {
+        let fuel_before = store.fuel_consumed().unwrap();
+        let output = match fibonacci.call(n) {
+            Ok(v) => v,
+            Err(_) => {
+                println!("Exhausted fuel computing fib({})", n);
+                break;
+            }
+        };
+        let fuel_consumed = store.fuel_consumed().unwrap() - fuel_before;
+        println!("fib({}) = {} [consumed {} fuel]", n, output, fuel_consumed);
+        store.add_fuel(fuel_consumed)?;
+    }
+    Ok(())
+}