Abstract timestamps with embedded-time

[tmplt]

https://crates.io/crates/embedded-time provides a lot of nice abstractions for raw timestamps. We probably want to replace Timestamp::base and Timestamp::delta with a embedded_time::Instant. The API needs to account for the clock frequency for the embedded_time::Clock we need to create.

[tmplt]

The frequency of the trace clock must be stored in an embedded_time::Clock. This will only be possible when const generics is stabilized:

use embedded_time as time;

// semi-pseudo
struct TraceClock<const Freq: usize>;
impl<Freq> time::Clock for TraceClock<Freq> {
    type T = u64;
    const SCALING_FACTOR: time::fraction::Fraction = <time::fraction::Fraction>::new(1, Freq);

    fn try_now(&self) -> Result<time::Instant<Self>, time::clock::Error> {
        // ...
    }
}

[Yatekii]

I am not so sure, having a constant frequency is a good choice. The ITM clock is not fixed. So we want to change this at runtime depending on the chip.

[tmplt]

fugit provides an alternative to embedded-time that supports const generics.