* include/wget/wget.h: New functions wget_hashmap_iterator_alloc(),
  wget_hashmap_iterator_free(), wget_hashmap_iterator_next(),
  also for stringmap.
* libwget/hashmap.c: Implementation of the above functions
* libwget/stringmap.c: Add wget_stringmap_iterator_next()
* unit-tests/test.c: Add tests using stringmap iterator

This functionality is sometimes more handy than the *_browse() API
with callback functions and context variables.