Provide an API to get reliable system uptime

[AnthonyGrondin]

Currently, there are multiple timers, hardware and software that can be used in multiple configurations on the ESP32. Some have edge cases, like xtensa_lx::timer::get_cycle_count() that overflows after a certain time, or Systimer that overflows too on certain targets.

There should be a unified public API that allows reliably fetching the current uptime of the board, no matter the timer(s) configurations. It could also implement traits from Fugit to get the uptime in multiple formats, such as Duration (ms, seconds, etc), and count (total cycle count).

See also: https://github.com/esp-rs/esp-wifi/blob/e8042d54b63c4807b3c66b5d8923e38d9eae6d8e/esp-wifi/src/lib.rs#L83-L86

[jessebraham]

I also would like to see something like this, so thanks for opening this issue. I'm pretty swamped currently, but I will see if I can find somebody to look into this in the next couple weeks.