Add svn_fs_move() to the FS API and support it in all backends.
Once at it, slightly reorder the FS vtable.

* subversion/include/svn_fs.h
  (svn_fs_move): declare new API

* subversion/libsvn_fs/fs-loader.h
  (root_vtable_t): add move() entry; shift generic copy functions
                   up to the generic node operations section

* subversion/libsvn_fs/fs-loader.c
  (svn_fs_move): implement by forwarding to the vtable

* subversion/libsvn_fs_base/tree.c
  (root_vtable): update vtable

* subversion/libsvn_fs_fs/tree.c
  (root_vtable): update vtable

* subversion/libsvn_fs_x/tree.c
  (root_vtable): update vtable

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1525442 13f79535-47bb-0310-9956-ffa450edef68