Add examples of linking and WASI (#1369)

* Add examples of linking and WASI

This commit adds two example programs, one for linking two modules
together and one for instantiating WASI. The linkage example
additionally uses WASI to get some meaningful output at this time.

cc #1272

* Add examples to the book as well

* More links!

* Ignore examples from rustdoc testsing

* More example updates

* More ignored

examples/linking.rs

@@ -0,0 +1,57 @@
+//! Example of instantiating two modules which link to each other.
+
+// You can execute this example with `cargo run --example linking`
+
+use anyhow::Result;
+use wasmtime::*;
+use wasmtime_wasi::{Wasi, WasiCtx};
+
+fn main() -> Result<()> {
+    let store = Store::default();
+
+    // Load and compile our two modules
+    let linking1 = Module::from_file(&store, "examples/linking1.wat")?;
+    let linking2 = Module::from_file(&store, "examples/linking2.wat")?;
+
+    // Instantiate the first, `linking2`, which uses WASI imports
+    let wasi = Wasi::new(&store, WasiCtx::new(std::env::args())?);
+    let mut imports = Vec::new();
+    for import in linking2.imports() {
+        if import.module() == "wasi_snapshot_preview1" {
+            if let Some(export) = wasi.get_export(import.name()) {
+                imports.push(Extern::from(export.clone()));
+                continue;
+            }
+        }
+        panic!(
+            "couldn't find import for `{}::{}`",
+            import.module(),
+            import.name()
+        );
+    }
+    let linking2 = Instance::new(&linking2, &imports)?;
+
+    // And using the previous instance we can create the imports for `linking1`,
+    // using the previous exports.
+    let mut imports = Vec::new();
+    for import in linking1.imports() {
+        if import.module() == "linking2" {
+            if let Some(export) = linking2.get_export(import.name()) {
+                imports.push(export.clone());
+                continue;
+            }
+        }
+        panic!(
+            "couldn't find import for `{}::{}`",
+            import.module(),
+            import.name()
+        );
+    }
+    let linking1 = Instance::new(&linking1, &imports)?;
+
+    // And once everything is instantiated we can run!
+    let run = linking1.get_export("run").and_then(|e| e.func()).unwrap();
+    let run = run.get0::<()>()?;
+    run()?;
+    Ok(())
+}