Switch to mocha/chai

This is not quite ready but... It is already way too large

The HTML version covers both the feature tests and the old unit tests. Run it from `test/index.html`

To run the unit tests (the same tests) in Node.js:

```bash
$ mocha --ui tdd -R spec --require test/unit/node-env.js \
    test/unit/{codegeneration,syntax,semantics,util,system}/ \
    test/unit/runtime/modules.js 
```

The Node.js feature tests are still run using make test. I'll fold that into mocha in a different PR later.

There are some hacks in here. I'll make comments inline about them.

Feedback wanted!