salad: rename a few bps_tree methods

 - Rename bps_tree_iterator_are_equal to bps_tree_iterator_is_equal for
   consistency with other methods that check two objects for equality
   (for example, tt_uuid_is_equal).

 - Rename bps_tree_iterator_first and bps_tree_iterator_last to
   bps_tree_first and bps_tree_last, because these are methods of
   bps_tree, not bps_tree_iterator. Omitting _iterator is also
   consistent with bps_tree_lower_bound and bps_tree_upper_bound
   methods, which also create bps_tree_iterator objects.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring