[whatwg/streams] Refactor exports (#899)

I'm working on an updated polyfill for streams over at [MattiasBuelens/web-streams-polyfill](https://github.com/MattiasBuelens/web-streams-polyfill). I use Rollup to convert the reference implementation from CommonJS to ES2015 modules, and then bundle it in a nice UMD wrapper.

This works great, but unfortunately the [Rollup CommonJS plugin](https://github.com/rollup/rollup-plugin-commonjs) fails to optimally transform some parts of the code. For example, it has trouble with this function in `helpers.js`:
```js
exports.createDataProperty = (o, p, v) => {
  assert(exports.typeIsObject(o));
  Object.defineProperty(o, p, { value: v, writable: true, enumerable: true, configurable: true });
};
```
Because `exports.typeIsObject` might be re-assigned externally, the plugin must retain the `exports` object and wrap the whole module with a `createCommonJS` call. This prevents further optimizations such as tree-shaking.

This PR changes all instances of `exports.foo = ...` with `const foo = ...; module.exports = { foo };` so that internal functions can use `foo` instead of `exports.foo`. This prevents externally re-assigned exports from changing the behavior of internal functions, and allows Rollup to transform the CommonJS module into a "proper" ES2015 module.

I realize that this is not immediately relevant for the reference implementation, but it helps to shave off a couple of bytes in the polyfill. I could keep this in my fork of the reference implementation, but I'd prefer not to need a fork. 😅 
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/899

-- Commit Summary --

  * Refactor exports

-- File Changes --

    M reference-implementation/lib/helpers.js (54)
    M reference-implementation/lib/queue-with-sizes.js (15)
    M reference-implementation/lib/utils.js (4)

-- Patch Links --

https://github.com/whatwg/streams/pull/899.patch
https://github.com/whatwg/streams/pull/899.diff

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