Rename RBTREE & BPTREE _clean in _reset to avoid confusing _clean & _clear functions. See #97

tests/test-mbptree.c

@@ -187,7 +187,7 @@ static void test2(void)
   assert (k == -2);
   r = btree_pop_at(NULL, b, 0);
   assert (r == false);
-  btree_clean(b);
+  btree_reset(b);
   assert (btree_size(b) == 0);
   assert (btree_empty_p(b));
   btree_clear(b);