Short-circuit transformSingle

There is no need to recurse in tree if there are no transformations left.
Short-circuit TreeTransform.transform

No need to perform zero transformations.
Fix wrong period for creating nested ctx in TreeTransforms.

`owner` for nestedContext in `TreeTransforms` was assigned
using initial period of whole `TreeTransforms` block.
That is incorrect as denotation for this owner could have been
create by some phase in this block.
Phases in tests now also have valid periods.
Merge pull request #142 from DarkDimius/fix-transforms-period

Fix using of wrong period for creating nested ctx in TreeTransforms