Fixed bug in treating Thickets in TreeTransform
New functionality: changeOwnerAfter

Changes owners after a phase without copying the tree. This should be more
suitable for the changeOwner operations used in the tree transforms so far,
which are linear, i.e. no tree duplication is needed.
Merge memoize with miniphases following it into one group.
Merge mixin with the group of miniphases following it.