JIT ("Just in time") compilation

This pull request adds a specialized class loader for the efficient "edit / save / run" paradigm at development time.

```sh
$ cat > HelloWorld.xp
public class HelloWorld {
  public static void main(string[] $args) {
    util.cmd.Console::writeLine('Hello ', $args.length ? $args[0] : 'World');
  }
}
$ xp HelloWorld
Hello World
```

The `xp.compiler.JitClassLoader` gets registered by adding `compiler/jit` to the `use` line instead of just `compiler`, which in turn adds `jit/init.php` to the class path, which then registers the `JitClassLoader` instance as class loading delegate. 

*Note: We need to use a small trick to register the class loader. This is what modules where designed for*