New functionality: changeOwnerAfter
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.

folone/dotty,VladimirNik/dotty,vsalvis/dotty,densh/dotty,vsalvis/dotty,dotty-staging/dotty,yusuke2255/dotty,angi2/dotty,svalaskevicius/dotty,som-snytt/dotty,densh/dotty,svalaskevicius/dotty,folone/dotty,AlexSikia/dotty,sjrd/dotty,auvik/dotty,som-snytt/dotty,sjrd/dotty,spetz911/dotty,som-snytt/dotty,angi2/dotty,auvik/dotty,dotty-staging/dotty,sjrd/dotty,reactormonk/dotty,reactormonk/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,VladimirNik/dotty,dotty-staging/dotty,som-snytt/dotty,yusuke2255/dotty,lampepfl/dotty,lampepfl/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,AlexSikia/dotty,spetz911/dotty,som-snytt/dotty,lampepfl/dotty