* Changelog: Summary of new functions.
* nih/hash.c: New functions:
  - nih_hash_foreach()
  - nih_hash_count()
* nih/hash.h: Prototypes for new functions:
  - nih_hash_foreach()
  - nih_hash_count()
* nih/list.c: New functions:
  - nih_list_foreach()
  - nih_list_count()
* nih/list.h:
  - Prototypes for new functions:
    - nih_list_foreach()
    - nih_list_count()
  - New type: NihListHandler.
* nih/tree.c: New functions:
  - nih_tree_foreach()
  - nih_tree_count()
* nih/tree.h:
  - Prototypes for new functions:
    - nih_tree_foreach()
    - nih_tree_count()
  - New type: NihTreeHandler.
* nih/tests/test_hash.c:
  - New functions:
    - test_count()
    - test_foreach_func () and supporting functions:
      - hash_handler1()
      - hash_handler2()
      - hash_handler3()
  - Updated main to call test_foreach_func() and test_count().
  - Changed description for existing functions to make it clear that
    NIH_HASH_FOREACH() is a macro.
* nih/tests/test_list.c:
  - New functions:
    - test_count()
    - test_foreach_func() and supporting functions:
      - list_handler1()
      - list_handler2()
      - list_handler3()
  - Updated main to call test_foreach_func() and test_count().
  - Changed description for existing functions to make it clear that
    NIH_LIST_*() are macros.
* nih/tests/test_tree.c
  - New functions:
    - test_count()
    - test_foreach_func () and supporting functions:
      - tree_handler1()
      - tree_handler2()
      - tree_handler3()
  - Updated main to call test_foreach_func() and test_count().
  - Changed description for existing functions to make it clear that
    NIH_TREE_FOREACH_*() are macros.