Submit clock API to embedded-hal

[hannobraun]

The clock API could potentially be completely platform-independent and live in embedded-hal. Before that can happen, we should review and improve the design.

Here are a few improvements I can think of:

• Frequency should have an associated type Freq. Frequency::hz would return Self::Freq instead of u32.
• There should be an additional trait, Clock, that defines an associated type, Ticks. This type would be used for value in the Ticks struct, as u32 is not appropriate for all platforms.

Maybe it would be a good idea to add some more use cases before the clock API is submitted. The current clocks are pretty simple, as they have a constant frequency. Some ideas for additional use cases:

• The external WKT clock, as that's fed into the microcontroller via a pin.
• The UART clock, as that needs to deal with configurable frequencies.