Update format of serialisation for B+TREE:

as the tree is ordered, a '[]' has to be used.

tests/test-mbptree.c

@@ -312,7 +312,7 @@ static void test_io(void)
   M_LET(str, STRING_OPLIST)
   M_LET(tree1, tree2, BPTREE_OPLIST2(btree_my, TESTOBJ_CMP_OPLIST, TESTOBJ_CMP_OPLIST)) {
     btree_my_get_str(str, tree1, false);
-    assert(string_equal_str_p(str, "{}"));
+    assert(string_equal_str_p(str, "[]"));
     const char *endp;
     bool b = btree_my_parse_str(tree2, string_get_cstr(str), &endp);
     assert(b);
@@ -339,7 +339,7 @@ static void test_io(void)
     btree_my_set_at(tree1, mpz1, mpz2);
 
     btree_my_get_str(str, tree1, false);
-    assert(string_equal_str_p(str, "{17:170,67:670}"));
+    assert(string_equal_str_p(str, "[17:170,67:670]"));
     b = btree_my_parse_str(tree2, string_get_cstr(str), &endp);
     assert(b);
     assert(*endp == 0);