(language) Add object data type

The perlang::Object class already existed on the C++ side, so we were able to utilize that class for this. However, it was previously not the base class for perlang::String and others, so this was also fixed now.

Objects of type object are not very usable right now, since we don't have any form of typecasting mechanism (and also no reflection that would enable some form of dynamic dispatch). They can be print()ed, and stored in instance fields and variables, but that's pretty much it right now.