Previously this exported a single function `undom()`, which had all the
classes and functions inside, and would create new classes (including
new prototypes, constructors, etc. for those classes) everytime you
create a new document, which is not very efficient to say the least.
So the first step to improve the performance here was to move all the
classes and functions out of the `undom()` function (which I took the
liberty to rename to `createDocument()` now, as that seems to make more
sense to me).

The next step was to move the methods and accessors that were slapped
onto the `Document` and `Element` instances to the respective prototypes
instead. Specifically we avoid installing the 'cssText' and 'className'
accessors in the constructor, and instead have these accessors installed
once on the `Element.prototype`, which gave a huge performance boost.

The execution time of the following micro-benchmark, which creates lot's
of documents and adds a single HTMLUnknownElement <app-root> to it, goes
from around 6690ms to 67ms, which is a solid **100x** performance
improvement.

```js
const createDocument = require('undom');

function render() {
  const d = createDocument();
  d.body.appendChild(d.createElement('app-root'));
  return d;
}

for (let i = 0; i < 1e2; ++i) render();

console.time('timer');
for (let i = 0; i < 1e5; ++i) render();
console.timeEnd('timer');
```

Note that this is a breaking change in the sense that prototypes will be
shared by multiple documents. But this seems to be consistent with
what other DOM libraries like jsdom and domino do.