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

crates/misc/run-examples/src/main.rs

@@ -2,13 +2,18 @@ use std::collections::BTreeSet;
 use std::process::Command;
 
 fn main() {
+    let example_to_run = std::env::args().nth(1);
     let examples = std::fs::read_dir("examples").unwrap();
     let examples = examples
-        .map(|e| {
+        .filter_map(|e| {
             let e = e.unwrap();
             let path = e.path();
             let dir = e.metadata().unwrap().is_dir();
-            (path.file_stem().unwrap().to_str().unwrap().to_owned(), dir)
+            if let Some("wat") = path.extension().and_then(|s| s.to_str()) {
+                return None;
+            }
+
+            Some((path.file_stem().unwrap().to_str().unwrap().to_owned(), dir))
         })
         .collect::<BTreeSet<_>>();
 
@@ -21,14 +26,24 @@ fn main() {
         if example == "README" {
             continue;
         }
+        if let Some(example_to_run) = &example_to_run {
+            if !example.contains(&example_to_run[..]) {
+                continue;
+            }
+        }
         if is_dir {
             println!("======== Rust wasm file `{}` ============", example);
+            let target = if example == "fib-debug" {
+                "wasm32-unknown-unknown"
+            } else {
+                "wasm32-wasi"
+            };
             run(Command::new("cargo")
                 .arg("build")
                 .arg("-p")
                 .arg(format!("example-{}-wasm", example))
                 .arg("--target")
-                .arg("wasm32-unknown-unknown"));
+                .arg(target));
         }
         println!("======== Rust example `{}` ============", example);
         run(Command::new("cargo")