Use embedded-time instead of custom implementation

[wbuck]

When writing a driver for some device (say a motor driver) and attempt to program against the trait's in the embedded-hal you can run into issues when using a peripheral that requires types defined in time.rs.

Because of this the driver now has a dependency on the stm32f3xx-hal device crate.
This of course is unwanted as it would be nice to create a generic device driver that doesn't depend on any specific device crate.

Example:

pub struct Driver<T>
where 
    T: CountDown
{ 
    timer: T,
}

impl<T> Driver<T> 
where 
    T: CountDown
{
    pub fn new( timer: T ) -> Self { Driver { timer }  }

    pub fn start( &self ) { 
         self.timer.start( /* This requires Hertz which is defined in time.rs of the stm32f3xx-hal crate */ );
    }
}

Instead of using the units/functionality defined in time.rs could the units/functionality defined in embedded-time be used instead? This would allow for a truly generic driver (assuming other device crates got on board).

This is probably more of community issue as a lot of device crates use their own custom time.rs implementation.

Below are two issues regarding this for the embedded-hal crate.
It sounded like the consensus was to have device crates get on board with using some centralized source for using time based units and functionality.

embedded-hal #221
embedded-hal #59

[wbuck]

Just noticed that there was already a request for this: #185

[Sh3Rm4n]

Closed by #192