Make Node.children default

1. This allows subclasses to narrow the type
2. Abstract subclasses like Expression need to have an own initializer
   parameter children, for which they must declare actual children

To fix the boundary between initializer and declaration section,
ceylonExpression_cache also had to be moved up.
Node: add TODO case types

I don’t want no one adding foreign Node types, thank you very much.
Add Expression

Not much there yet, but now I can build up the expression hierarchy
bottom-up.
Add IntegerLiteral (closes #11)
Add tests for IntegerLiteral

Note: the tests with uppercase hex literals require
ceylon/ceylon.language#468 to pass.