Async For/Of

[zenparsing]

We are currently using the function context to determine whether to make for/of async-compatible. We need to use explicit syntax for this.

For now, let's try using something like:

for async (let item of collection) {
    // ...
}

For the implementation, let's try this:

• Try to get an async iterator using obj[Symbol.asyncIterator]
• If that didn't work, then try to get a sync iterator with obj[Symbol.iterator].
• If that worked, then wrap the sync iterator with an async interface (this operation should be factored out).

[zenparsing]

Done