Implement generics/parameterized records

At least something like:

```
// Doubly-linked list
List: record<Type> {
   entry: Type
   prev: List<Type>
   next: List<Type>
}

list  = new List<int>
list.entry = 3
list.prev = null
list.next = null

second = new List<int>
second.entry = 4
second.prev = list
list.next = second
```

and it Just Works.