Function types

This pull request implements xp-framework/rfc#286 and adds function types:

* [x] `lang.FunctionType` basic implementation
* [x] `lang.Type::forName()` support
* [x] Parsing from api docs
* [x] `typeof()` integration
* [x] Implement cast() and isAssignableFrom()
* [x] Decide what to do with `FunctionType::newInstance()`