Improve API for switching to low-power modes

[hannobraun]

The current Pmu API only support switching to sleep mode. All other low-power modes should be supported as well.

I think there's also some potential for embedded-hal here. I'd like to expore the possibility of creating a trait, PowerMode, and implementing this trait for each supported power mode.

That trait could live in embedded-hal. The sleep module could be updated to depend on PowerMode instead of depending on Pmu directly.

[hannobraun]

There's a problem with LowPowerClock: It starts with clock::state::Disabled, but that may be wrong, as we can't know what happened before the HAL was initialized.

I'm not sure if that leads to any actual problems, but it's at the very least misleading. Maybe it would be best to add a third clock state, Unknown, and start with that.

Edit: This comment has been resolved in the meantime.

[hannobraun]

I've added support for additional low-power modes in #106.

[hannobraun]

Before trying to design anything that can be submitted to embedded-hal, at least #108, #109, and #110 should be resolved.