Xous Api: susres

This crate defines the Xous suspend/resume API.

The suspend/resume API is responsible for sequencing events before a system powers down, such that all of the SoC hardware blocks will lose state, but the RAM will retain its contents.

Every service that contains hardware state which can be lost when the SoC is powered down must register with the sequencer, and it must atomically back up its hardware state to battery-backed RAM and then call suspend_until_resume, which will block until power is restored. Once power is restored, the service should copy the hardware state out of battery-backed RAM and resume operations.

The API also contains hooks for initiating the suspend process, and for rebooting the device.