Deprecating svn_fs_id_t

For some time now, this thing has been a nuisance to me
in various situations. Recent discussions prompted me
to have a closer look at this and now I want to see it
removed from the API (maintaining it in the deprecated
section). Here is why; further down is how.

Why

---
svn_fs_id_t is ill-conceived and unnecessary concept with
an under-developed API:
* Ill-conceived - as this is not used as an ID / key to
  anything in the rest of the API.  The API looks like
    get_infoA(root, path)
    get_infoB(root, path)
  instead of
    get_ID(root, path)
    get_infoA(id)
    get_infoB(id)
  So, we rather exposed an implementation detail and had
  to deprecate svn_fs_parse_id() early on to prevent the
  most severe consequences.
* Unnecessary - as it gets in the way instead of actually
  contributing to an operation.  We use svn_fs_id_t to test
  for noderev identity and relationship but all code uses
  the following pattern:
    id_a = get_ID(rootA, pathA)
    id_b = get_ID(rootB, pathB)
    compare(id_a, id_b)
  Shorter and in keeping with the rest of the API, we should
  simply do
    compare(rootA, pathA, rootB, pathB)
  There are two more uses.  One for "debugging" (svnlook)
  which could use a similar private API call.  The other
  one is as a basis for an optimization and there is an
  alternative implementation for that.
* Under-developed - as it does not provide the API that
  it could.  For instance, it might return a root object
  (telling us the txn / rev that the noderev was created in).
  There is also no check on the copy id sub-struct that
  might answer a question like:  refer these noderevs to
  the same node with no copy operation in between?
    id_a = get_ID(rootA, path)
    id_b = get_ID(rootB, path)
    if (same_node(id_a, id_b) && same_copy(id_a, id_b))
      doIt()
  Note that the IDs in this scenario are still derived
  from Subversion's versioned file system semantics and
  independent from any concrete implementation.
How / what
-----------
* Introduce a root+path based svn_fs_compare and deprecate
  svn_fs_compare_ids as well as svn_fs_check_related.
* Deprecate svn_fs_dirent_t and replace it with svn_fs_dirent2_t
  containing no ID member.  Bump svn_fs_dir_entries API as well.
* Similarly bump svn_fs_path_change2_t plus related API.
* Deprecate svn_fs_unparse_id and svn_fs_node_id.  Introduce
  svn_fs__unparse_id and svn_fs__node_id as private API for
  debugging and tools like svnlook.  Also, use these to
  implement the deprecated directory and changed path list APIs.
-- Stefan^2.