Expression fields need to be marked as Child or Children

[TODO-REPORT: make clear that this is essential for replacing nodes/specialization, and that arrays need to be marked with @Children instead of @Child]

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Make SequenceNode optional

- do not use it if only one expression would be in it
- move the restart handling of loops directly into Method
- added missing calls to adoptChild()
[TODO-REPORT: all @Child fields should be assigned with the result of adoptChild()]

Signed-off-by: Stefan Marr <git@stefan-marr.de>