Revision 1888557

Incorporate danielsh' review feedback on the initial svn_task__* code.
This is improving naming, docstrings and assertions.

* subversion/include/private/svn_task.h
  (svn_task__process_func_t,
   svn_task__output_func_t): Clarify that the processing function output
                             is just temporary data.

* subversion/libsvn_subr/task.c
  (root_t,
   output_t,
   svn_task__t): output_t and related pools and instances shall be called
                 results_t etc. to avoid confusion with processing function
                 output blobs.
  (svn_task__t): Fix confusion and partially wrong docstrings.
  (first_ready_sub_task_idx): Improve docstring, commentary and complete
                              assertions.
  (link_new_task): Assert invariants for new tasks.
  (ensure_output): Renamed to ...
  (ensure_results): ... this and adapt to renamed structs & members.
  (add_task): Adapt to renamed structs & members.
              Add assertions to catch coding problems early.
  (free_sub_tasks): Replaced by ...
  (clear_errors): ... this and update docstring & logic.
  (process,
   execute_serially,
   svn_task__run): Adapt to renamed structs & members.
  (output_processed): Ditto.  Try to explain what "post-order" means here.