TimeDelta::from_now ensures the execution is within critical section

[trnila]

In my stm32 app under the load, the deltas are sometimes corrupted - see us_timeticks values jumped compared to logged get_tracing_raw_ticks for tick 9 and 10. The defmt_timestamp_s is also correct.

#[task]
async fn test_task() {
    let mut ticker = Ticker::every(Duration::from_millis(100));
    let mut i = 0;
    loop {
        ticker.next().await;
        defmt::info!("Tick {} {}", i, unsafe { get_tracing_raw_ticks() });
        i += 1;
    }
}

    uc_timeticks  defmt_timestamp_s           message        diff
1         266957               0.10     Tick 0 266908         NaN
2         532585               0.20     Tick 1 532533   265628.00
3         798209               0.30     Tick 2 798158   265624.00
4        1063835               0.40    Tick 3 1063783   265626.00
5        1329457               0.50    Tick 4 1329408   265622.00
6        1595085               0.60    Tick 5 1595033   265628.00
7        1860709               0.70    Tick 6 1860658   265624.00
8        2126335               0.80    Tick 7 2126283   265626.00
9        2391959               0.90    Tick 8 2391908   265624.00
10      69766448               1.00    Tick 9 2657533 67374489.00
11     137140935               1.10   Tick 10 2923158 67374487.00

The TimeDelta::from_now function must execute atomically within a critical section, as it modifies the global variable LAST_TIMESTAMP, which could lead to corrupted time deltas.

Requiring CriticalSection in the function arguments ensures the caller is in critical section and its not used incorrectly.