Macro big_o_test::assert_complexity

macro_rules! assert_complexity {
    ($observed_complexity:expr, $expected_complexity:expr $(,)?) => { ... };
    ($observed_complexity:expr, $expected_complexity:expr, $($arg:tt)+) => { ... };
}

experimental/rudimentary assertion macro to let an ‘observed_complexity’ better than ‘expected_complexity’ to pass, in the hope to reduce false-negative test failures