Improve sleep API

[hannobraun]

The current sleep API feels a bit icky. Some ideas for improvement:

• Add prepare method to trait, use associated type for parameters.
• Provide module-level sleep function for convenience, so the user doesn't need the two-step sleep.
• Deep power-down mode resets the microcontroller upon wake-up. This means a sleep function based on that mode needs to return ! instead of (). The Sleep trait should have an associated type to define the return type.
• Think about Clock: Should it be an associated type instead of a type parameter?

Once the API has matured a bit, it can be submitted to embedded-hal.

[hannobraun]

Two notes:

• Before submitting this to embedded-hal, it would make sense to submit a power mode API first, as that's lower-level. See Improve API for switching to low-power modes #3.
• I'm not sure if this is suitable for embedded-hal at all, as a sleep API is blocking by nature. It might be undesirable to have anything beyond timers and power modes in there.

I think it's best to defer this until a power mode API has been fleshed out and submitted, then think about this issue later on.

[hannobraun]

I'm removing the upstream label. I aleady expressed doubts on whether this should be submitted to embedded-hal, and in addition, embedded-hal has gained a sleep API in the meantime.

If we come up with something worthwhile, we can think about submitting it to embedded-hal later. For now, this issue is only about improving the sleep API in this crate.