moveInto support

This pull request solves issue #444.

It allows Collections to say "you can move a node into me". By giving collections the option to opt-in to this behavior, there are cases where move changes from a super slow directory-traversal algorithm, to a hyper-fast 'rename()' operation in the operating system.

This PR also removes Sabre\DAV\ObjectTree and Sabre\DAV\Tree\FileSystem. The main use-case for having different objects was overriding `move()`, and I haven't really heard of other people doing stuff with this extension point.
Should a path like `/` also be considered "the root"?