Re: [whatwg/dom] Consider specifying document.evaluate and document.createNSResolver (#67)

FWIW, I've just refactored some code from this:
```js
function notifyIfMatchesXPath(query, callback) {
  const flag = XPathResult.ORDERED_NODE_SNAPSHOT_TYPE;
  const callback = () => {
    const result = document.evaluate(query, document, null, flag, null);
    for (let i = 0, {snapshotLength} = result; i < snapshotLength; i++)
      callback(result.snapshotItem(i));
  };
  new MutationObserver(callback).observe(
    document,
    {characterData: true, childList: true, subtree: true}
  );
  callback();
}
```

to this:
```js
function notifyIfMatchesXPath(query, callback) {
  const evaluator = new XPathEvaluator();
  const expression = evaluator.createExpression(query, null);
  const flag = XPathResult.ORDERED_NODE_SNAPSHOT_TYPE;
  const callback = () => {
    const result = expression.evaluate(document, flag, null);
    for (let i = 0, {snapshotLength} = result; i < snapshotLength; i++)
      callback(result.snapshotItem(i));
  };
  new MutationObserver(callback).observe(
    document,
    {characterData: true, childList: true, subtree: true}
  );
  callback();
}
```

assuming the cost of parsing, and validating, the XPath query would've removed from the mutation dance/equation, but I've discovered only recently `XPathEvaluator` and `createExpression`, and I'm sure if other developers new about it, its usage would be closer to the `document.evaluate` one, which is also now at 2.x%.

If the direction is to nuke `XPathEvaluator` though, I rather would like to know it before landing such refactoring, thanks.

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