Add support for annotations improving minification and rewriting

Eg. ability to declare "pure" functions:

```
class Bar {
/* babili: no-side-effects */
constructor() {
someFunction(this);
}
}
new Bar();
```

Would result in removing `new Bar()` because of `no-side-effects` annotation.
Other possible annotations include: `dont-modify-argument`, `constant-expression`, `always-new` (function always return unique object - like `foo = ()=>({})` - all calls to `foo()` results in new objects) or similar.