[1/3] Timer refactor: system timer

[MabezDev]

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Submission Checklist 📝

• I have updated existing examples or added new ones (if applicable).
• I have used cargo xtask fmt-packages command to ensure that all changed code is formatted correctly.
• My changes were added to the CHANGELOG.md in the proper section.
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Pull Request Details 📖

Description

A fairly radical change, but I realized that it's basically impossible to treat the Unit's as a resource and allow users to actually make modifications (FrozenUnit prevented this before). Unit operations now go through the SystemTimer struct, and are all unsafe with safety comments explaining to the user how to use it correctly. Opening early for feedback before I update all the examples.

TL;DR:

• Removes all type params on Alarm
• Systimer no longer implements peripheral ref, the peripheral ref
  pattern is instead intended to be used on the higher level timer
  drivers (OneShot, Periodic)
• Removed Unit as a type, in favour of an enum
• Alarms are back in the main SystemTimer "driver"
• Made all Unit modification methods unsafe, it's not possible to
  modify the Unit's safely whilst timers and or the time::now API is
  in use

[bjoernQ]

I welcome the simplification here especially since we are encouraging users to just use the higher-level timers and use the hw-timers to back them

I don't think users did anything but use split before in which case all the generics were just annoying best case.

Not sure if even the now unsafe functionality is useful, but it probably doesn't hurt to have it (while it would hurt to use it wrong - that's why it's unsafe)

[bjoernQ]

Unrelated but I think the cycles function in examples/src/lib.rs is unused and t.b.h. it should - the examples should be self contained (I remember the hate the example-simplification-macros in esp-wifi got back then)

[MabezDev]

Another question I'm not sure about, should Alarm's be ZSTs? I.e I erased the alarm number to simplify AnyTimer, but maybe that was a step too far for a "dumb timer"?

[Dominaezzz]

Another question I'm not sure about, should Alarm's be ZSTs? I.e I erased the alarm number to simplify AnyTimer, but maybe that was a step too far for a "dumb timer"?

The goal is simplicity right? ZSTs add more type complexity, which goes against that.
Unless there's something you're concerned about losing by not using ZSTs.

[MabezDev]

Skip changelog for embassy and esp-wifi changes, whilst technically breaking, they're both tied to esp-hal releases so it won't be breaking when they upgrade

[bjoernQ]

LGTM

[playfulFence]

Thanks!

[jessebraham]

A few very nitpicky comments which you can feel free to ignore, but otherwise LGTM

[bugadani]

Thanks