Add mock for timers

[niclashoyer]

Added a mock for timers using embedded-time with nanosecond precision. A single clock can be used to create multiple timers. The clock can be freely skipped by any time value supported by embedded_time (e.g. microseconds, seconds, nanoseconds, …).

To support sharing a clock and timer between threads I relied on Arc<AtomicU64> for the time base. That also allows each timer to actually own the clock, so that they can be freely shared with any driver implementation.

I added this on top of #39. If wanted, I could also try to backport this.

[niclashoyer]

I think embedded_time needs a newer rust version. They don't have a MSRV given, so we need to figure that out somehow.

Edit: 1.51 seems to be right. It is now possible to also set the MSRV as package attribute in Cargo.toml since rust 1.56. This will generate a warning in versions < 1.56, but will not fail, see here.

[dbrgn]

Sorry for the long delay. I'm quite busy at the moment (as always 😂), so if someone else with timer experience would do a first review, that would already be helpful. (Anyone can do a review, not just maintainers 🙂)

The branch also needs a rebase.

[eldruin]

In the mean time we have decided to remove timers in embedded-hal 1.0 so I think it would not make much sense to merge this as-is.
Something that would make sense, though, would be to adapt the mock proposed here to the version available in embedded-hal 0.2.7. Pretty much cherry picking commit b8e03b8 on top of master would work as the interface was the same.
From a quick look the mock proposed here seemed fine but I can do a proper review of this afterwards.
One additional thing to think about would be whether to use embedded-time or fugit (or both).

[dbrgn]

@niclashoyer would you be interested in doing the backport suggested by @eldruin?

[niclashoyer]

@dbrgn @eldruin done! I'm currently moving to embedded-hal-async, but I think it still makes sense to backport this.

[dbrgn]

Sorry for the long delay again.

I rebased your branch against current master and will now review the contribution 🙂

Regarding the added dependencies, I'm thinking about putting the timer feature behind a cargo flag.

[dbrgn]

I pushed two changes:

• For consistency with other modules, I've renamed SimClock to MockClock and SimTimer to MockTimer.
• I put the module behind a Cargo feature (enabled by default)

Thanks! 👍🏻