Three new methods to support iteration over the keys stored in the
dictionary:

  * setFirst: set start of iterator to begin og dictionary
  * setLast: set start of iterator to end of dictionary
  * iter(dir): iterate over keys in dictionary in dir direction

The key pointer type t_dict_key_ptr is now moved to the public part
of the DictP package to support using in testbench code when saving the
keys with different lengths from iter() function.
Error return values are now of type t_dict_error with 3 possible values:
(NO_ERROR, KEY_INVALID, KEY_NOT_FOUND)