Making the use of scala.collection classes uniform throughout the project.

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@1617 85d21ef8-add0-11de-ae77-5bfb9c5a3f34
Removing mutable structures from cut-introduction.

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@1618 85d21ef8-add0-11de-ae77-5bfb9c5a3f34