add undo + redo
restructure tests
test_bline_delete
test_bline_insert; test_bline_get_col
test_buffer_add_mark; test_buffer_add_srule
test_buffer_delete; test_buffer_destroy; test_buffer_get; test_buffer_get_bline; test_buffer_get_bline_col; test_buffer_get_offset
test_buffer_add_srule; test_buffer_destroy; test_buffer_insert; test_buffer_new; test_buffer_redo; test_buffer_remove_srule
test_buffer_set; test_buffer_substr; test_buffer_undo