Made some more changes to the way namespaces are handled, for optimization purposes. This is a bit of a structural reorganization: $paths->pages is obsoleted in its entirety; calculating page existence and metadata is now the job of the Namespace_* backend class. There are many things in PageProcessor that should be reorganized, and page actions in general should really be rethought. This is probably the beginning of a long process that will be taking place over the course of the betas.