Add union types

[mfichman]

Similar to Haskell data structures. For example, a union type could be:

Json::Value = Bool | Int | Float | String | Array[Json::Value] | Map[Json::Value]

A union type is a subtype of any of the types in the union. Also, any type in the union is a subtype of the union type. Thus assignments such as this are legal:

val Json::Value = true
int Int = val

If 'val' is not the requested type, nil or the default value is used. Also, the is operator should be implemented:

if val is Bool {
    bool Bool = val
}

The is operator returns true if the tag is equal to the requested type. Every union has a tag that is equal to the vtable for the requested type:

struct Union {
    Ptr _vtable;
    Ptr _refcount;
    T _val ?
};

If the Union is a primitive or value, then the value is embedded directly into _val. If Union is a reference object, Union is just a pointer to the reference object. When packing/unpacking Unions, the _vtable field is examined to determine the type. So an assignment with a Union looks like this (in C):

int Int = (val._vtable == Int__vtable) ? val_.val : 0;
string String = (val._vtable == String__vtable) ? val : 0;

Union types are reference objects.

[mfichman]

Added any types, which is the first step in this process. Need to add boxing and type checking for unions.

[mfichman]

Complete.