Await/Defer Statement

The following section should be added to the language reference:

Contrary to JavaScript, any call to an async function is implicitly awaited in Wing. As
a result, await in Wing is a rarely used keyword, since its use is implied by
the inflight keyword. await is only used when you want a defered Promise
to be fulfilled before execution flow continues.
The Wing compiler emits awaits when encountering Promise<T> types as r-values
in expressions. Use the defer keyword to defer the resolution of a promise and
obtain a Promise<T> type instead (a.k.a unawait what the compiler does).

"defer" prevents the compiler from awaiting a promise and grabs a reference.
"await" and "Promise" are semantically similar to JavaScript's promises.
"await" statement is only valid in inflight function declarations.
Awaiting non promises in Wing is a no-op just like in JavaScript.

// Wing program:
class MyClass {
  inflight foo(): num {
    let w = defer somePromise();
    let x = await w;
    return x;
  }
  inflight boo(): num {
    let x = somePromise();
    return x;
  }
}

No file or test is named; start by locating the language reference and review the existing sections for async functions, inflight declarations, and Promise behavior. Add the supplied Await/Defer Statement section with its Wing example, and consider the work done when the reference explains the stated await, defer, and implicit-await semantics.