Make sure the period of a context always contains a single phase

Multi-phase periods mean that ctx.phaseId is no longer valid, which means
a lot of invariants break.
Ensure that transformStats is run at phase thisTransformPhase.
Make sure AnnotationTransformers run at own phase
Change treeTransformPhase for SyntheticMethods to new default
Change phase handling in enteredAfter

This makes it more consistent with related methods ensureFreshScopeAfter,
installAfter.
Make Literalize run at new default phase thisTransform.next
Change default treeTrannsformPhase to phase.next
Eliminate GettersSetters

Has been replaced by Getters, but some traces remained.
Remove fruitless overrides of treeTransformPhase

Now the default phase of treeTransformPhase is phase.next, we
can remove all overrides that specify the same phase.
Merge pull request #286 from dotty-staging/change/defaultphase

Change/defaultphase