Add pulseio.PulseIn support

[tannewt]

I can be bitbanged in a separate thread or process. Bitbanging code is here:

adafruit/Adafruit_CircuitPython_DHT#18

[ladyada]

this is almost done using libgpiod

[ladyada]

this is done in libgpiod but we'll need to do more testing on various boards and also figure out how to compile the pulsein exec or whether we'll include it.

[makermelissa]

This is in place, but as stated in the previous comment, more boards (at least ones with libgpiod support) will need testing and putting in place.

[makermelissa]

Fixed via #506 and #593. The pulsein library is included and the appropriate file is automatically detected and loaded.