Provide factory methods for simple refs (primitives)

[leventov]

Currently TypeFactory has a lot of static fields like INTEGER, INTEGER_PRIMITIVE, etc. But because any object cannot appear in AST twice, it's often requred to write code like

factory.Field().createField(factory.Core().clone(factory.Type().INTEGER_PRIMITIVE), "foo", ...)

when constructring some AST from stratch, i. e. just clone a constant. (And it's very easy to forget to do that.)

I suggest to add factory methods like TypeFactory.integer(), TypeFactory.integerPrimitive() which create a unique object referencing Integer, int, etc.