Fix bug with returns without expression

Of course, to return from a void function, a return without a return
value/expression is perfectly legal.
Implement Exists, Nonempty

With this, FormattingVisitor.ceylon can successfully be formatted.
Running the FormattingVisitor with and over the formatted code
introduces no further changes, which means that the formatter produced
valid code. There are still some ugly things in the output – and of
course, the FormattingVisitor doesn’t use all language features, so the
formatter isn’t complete yet – but this is still a big step.