Implement the embedded_hal_async::delay::DelayUs trait

[jessebraham]

This trait as added in one of the alpha releases, and we never implemented it. This trait will need to be implemented in order to use future versions of embassy.

• SYSTIMER (Implement embedded_hal_async::delay::DelayUs trait for SYSTIMER alarms #812)
• TIMG
• Xtensa CPU Timers (?)

[MabezDev]

This trait has been around since the 0.1 release of the eha, but didn't really need to implement it as embassy-time offers an impl based on the time driver impls. If we were to implement this we should impl on the following

• Systimer (alarms only)
• TIMG timers
• Xtensa internal CPU timers (maybe)
• Anything else that has a configurable time period + interrupt? Maybe something in the RTC domain

[jessebraham]

Once #1063 is resolved we can revisit this issue.

[jessebraham]

This trait is implemented for SYSTIMER directly, and is additionally implemented for OneShotTimer; since the timer::timg module still does not contain an asynch module of its own, I'm just going to close this as "completed".