[3/3] Timer abstraction: cleanup, simplification, and documentation

[jessebraham]

This should wrap up my work on the timer abstraction layer. This PR still needs testing, as I have changed some APIs a bit and as such they need confirmation. However, I think everything should be in okay shape.

The documentation definitely could use some improving, but I was struggling to come up with what more I could write or some better examples. Open to any input here, but perhaps that can be improved in another PR by somebody else instead.

I don't think this really needs a CHANGELOG.md entry, but let me know if I've missed anything; happy to add one if needed.

Closes #1479.

[MabezDev]

Thanks for finishing this up! I really liked how this turned out, hopefully it will make some future abstractions in esp-wifi etc a bit easier.

I still need to do some testing tomorrow, but I noticed one thing regarding the load_value checks for systimer.

[jessebraham]

Really questioning the value of clippy these days... 😓

[MabezDev]

Thank you! LGTM!

[bjoernQ]

Thanks! Code looks good to me and I did some tests on a previous commit - wouldn't expect anything bad from the latest commits. Looking forward to use the timer abstraction in esp-wifi later