Optimise machine.time_pulse_us implementation for code size, and switch esp8266 port to use it

[dpgeorge]

Summary

The esp8266 has a custom implementation of machine.time_pulse_us(), which is needed so it can feed the WDT during long waits for a pin change. It doesn't need to have a separate implementation, instead we can use the common one with an optional hook for a port to do work during the busy wait loops.

This PR:

1. Reworks the common implementation of machine.time_pulse_us() to be more like the esp8266 version. That cuts down code size a little, and improves accuracy a little.
2. Switches esp8266 to use this common implementation.

Also, motivation comes from #16160: this PR should make that PR much simpler (and lower cost) to implement.

Testing

Tested with a range of square waves from 125Hz to 4kHz on:

• PYBD_SF2: this has a very accurate measurement and accuracy is unchanged with this PR
• RPI_PICO2_W: same as PYBD_SF2, very accurate and unchanged with this PR
• ESP8266: for the frequencies it can handle (up to about 1kHz) this PR is slightly more accurate than before: for example a pulse that is 1000us long, it would measure between 998 and 1000us with an average of 999us. Now it measures between 999us and 1001us with an average of 1000us.

Also tested the timeout argument, a value of 5000000 still takes 5s and doesn't lead to a WDT timeout on esp8266.

Trade-offs and Alternatives

This is an optimisation. There is no change in the API. As tested above, the accuracy is at least the same if not better due to better sampling of ticks and pin at the same point in the code.

[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:   +32 +0.004% standard
      stm32:   -16 -0.004% PYBV10
     mimxrt:   -16 -0.004% TEENSY40
        rp2:    -8 -0.001% RPI_PICO_W
       samd:   -24 -0.009% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32

[dpgeorge]

It's really hard to pass CI these days 😅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (5676b45) to head (398da22).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #17346   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
=======================================
  Files         169      169           
  Lines       21941    21946    +5     
=======================================
+ Hits        21621    21626    +5     
  Misses        320      320           

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[dpgeorge]

@robert-hh regarding your comment that I think you deleted: it should be possible to allow nchanges to be any value, but the initial pulse_value would need to be twiddled like: pulse_value ^= nchanges & 1. Ie if there are an odd number of changes, toggle the initial value.

[robert-hh]

Yes, I delete it because at second though it was wrong. The nchanges counter can not underrun.
Tests with a SAMD21 also returned improved accuracy. A 1000µs pulse showed most values being either 999 or 1002, and an average over 100 samples of 1000 +/- 0.15.

[projectgus]

Nice cleanup!