Update WASI example to use shared Wasi::add_to_config.

This commit updates the WASI example to use `Wasi::add_to_config`.

As only a single store and instance are used in the example, it has no semantic
difference from the previous example, but the intention is to steer users
towards defining WASI on the config and only using `Wasi::add_to_linker` when
more explicit scoping of the WASI context is required.

examples/wasi/main.rs

@@ -14,20 +14,25 @@ fn main() -> Result<()> {
         .with_ansi(true)
         .init();
 
-    let store = Store::default();
-    let mut linker = Linker::new(&store);
+    // Define the WASI functions globally on the `Config`.
+    let mut config = Config::default();
+    Wasi::add_to_config(&mut config);
+
+    let store = Store::new(&Engine::new(&config)?);
 
-    // Create an instance of `Wasi` which contains a `WasiCtx`. Note that
-    // `WasiCtx` provides a number of ways to configure what the target program
+    // Set the WASI context in the store; all instances in the store share this context.
+    // `WasiCtxBuilder` provides a number of ways to configure what the target program
     // will have access to.
-    let wasi = Wasi::new(
+    assert!(Wasi::set_context(
         &store,
         WasiCtxBuilder::new()
             .inherit_stdio()
             .inherit_args()?
-            .build()?,
-    );
-    wasi.add_to_linker(&mut linker)?;
+            .build()?
+    )
+    .is_ok());
+
+    let mut linker = Linker::new(&store);
 
     // Instantiate our module with the imports we've created, and run it.
     let module = Module::from_file(store.engine(), "target/wasm32-wasi/debug/wasi.wasm")?;