Split code example into individual match cases; add code for example

blog/content/second-edition/posts/12-async-await/index.md

@@ -387,12 +387,6 @@ TODO
 
 
 ```rust
-impl Future for ExampleStateMachine {
-    type Output = String; // return type of `example`
-
-    fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
-        loop {
-            match self { // TODO: handle pinning
 ExampleStateMachine::Start(state) => {
     // from body of `example`
     let foo_txt_future = async_read_file("foo.txt");
@@ -403,6 +397,8 @@ impl Future for ExampleStateMachine {
     };
     *self = ExampleStateMachine::WaitingOnFooTxt(state);
 }
+```
+```rust
 ExampleStateMachine::WaitingOnFooTxt(state) => {
     match state.foo_txt_future.poll(cx) {
         Poll::Pending => return Poll::Pending,
@@ -423,6 +419,8 @@ impl Future for ExampleStateMachine {
         }
     }
 }
+```
+```rust
 ExampleStateMachine::WaitingOnFooTxt(state) => {
     match state.bar_txt_future.poll(cx) {
         match state.bar_txt_future.poll(cx) {
@@ -435,12 +433,19 @@ impl Future for ExampleStateMachine {
         }
     }
 }
+```
+```rust
 ExampleStateMachine::End(_) => {
     panic!("poll called after Poll::Ready was returned");
 }
-            }
-        }
-    }
+```
+
+
+```rust
+fn example(min_len: usize) -> ExampleStateMachine {
+    ExampleStateMachine::Start(StartState {
+        min_len,
+    })
 }
 ```