Async support

[Dion Dokter]

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