Consider whether algebraic data types should use sealed abstract classes

[noelwelsh]

There are several benefits to using a sealed abstract class as the base of an algebraic data type:

• it is faster
• Java interop is cleaner
• it is semantically clearer (an abstract class can't be mixed into multiple classes)

The disadvantage is we have to introduce another concept (abstract classes).

This will require quite a bit of rewriting.