Async support

[diondokter]

Now that GATs are stable, async support can be added on stable. This would really help when writing async code.

Some of the APIs that would benefit from this are:

embedded-time/src/clock.rs

Line 48 in 83b09cb

fn try_now(&self) -> Result<Instant<Self>, Error>;

embedded-time/src/timer.rs

Line 115 in 83b09cb

pub fn elapsed(&self) -> Result<Dur, TimeError>

embedded-time/src/timer.rs

Line 140 in 83b09cb

pub fn remaining(&self) -> Result<Dur, TimeError>

embedded-time/src/timer.rs

Line 163 in 83b09cb

pub fn wait(self) -> Result<Timer<'a, OneShot, Armed, Clock, Dur>, TimeError> {

The async APIs should probably be made parallel to the existing APIs and not replace them