Add `.throw` and `.return` interface methods to StreamToAsyncIterator

[basicdays]

Generators can also provide methods to implement a return and throw feature to help handle additional syntax operators in a for-of loop.

For example:

const readStream = fs.createReadStream(__filename);
    for await (const chunk of new S2A(readStream)) {
        console.log(chunk);
        if (chunk === ...) break;
    }

This is how it is used within a vanilla generator:

function* gen(){
  try {
    for (var i = 0;; i++) yield i;
  } finally {
    console.log('done');
  }
}

for (var i of gen()) {
  if (i > 5) break;
}

[basicdays]

This is a decent mention of how these features could work in an interator: http://jsrocks.org/2015/09/javascript-iterables-and-iterators/#optional-iterator-methods

This is how Babel transpiles generators to use the optional methods: http://babeljs.io/repl/#?babili=false&evaluate=true&lineWrap=false&presets=es2015&experimental=false&loose=false&spec=false&playground=true&code=%0Afor%20(var%20i%20of%20gen())%7B%0A%20%20%0A%7D&runtime=true