Smalltalk commit:

* Reorganized the classes in order to remove categories with only one class.
* Created UziPrimitive, which will hold a reference to all the valid primitives in the system.
* Removed primitive stuff from Uzi class.
* Changed all places where the creation of an UziPrimitiveCallBytecode used a number or a symbol as an argument. Now they must be an UziPrimitive instance.
* Changed UziInterpreter to work as a visitor, removing some ugly switch statements.