This PR introduces a new concept inside Expr's type checker - Nature.
It's an abstraction other reflect.Type which carries additional information about possible type, methods and builtin function.
I was able to find and fix a few bugs in type checker and optimizer. Implementation of nature concept has proven itself. As well nature will allow to bring more feature for type checker in future.
Why Nature? I was looking for a name to replace Type. Type name already carries much information, as well can be confused with reflect.Type. Nature name should be understood as "nature of the node", same as "type of the node".
