Bootstrap optimization

This pull request aims at optimizing the bootstrapping phase by loading less code and using fewer filesystem calls.

The difference is quite small, but can be seen (your mileage will vary):

* * * 

### Bootstrapping from file system
```sh
[master] $ xp -w '"Hello Wörld"'
Hello Wörld
Time: 0.030 seconds

[bootstrap-opt] $ xp -w '"Hello Wörld"'
Hello Wörld
Time: 0.028 seconds
```

### Bootstrapping from .xar file
```sh 
[master] $ xp -w '"Hello Wörld"'
Hello Wörld
Time: 0.027 seconds

[bootstrap-opt] $ xp -w '"Hello Wörld"'
Hello Wörld
Time: 0.024 seconds
```

Measured by adding the following to the top of `tools/class-main.php`:

```php
register_shutdown_function(function() {
  fprintf(STDERR, "Time: %.3f seconds", microtime(true) - $_SERVER['REQUEST_TIME_FLOAT']);
});
```

* * * 

### Why I vote to include this
Better than the slight performance optimization is the fact that this change actually makes the code more readable.