Add new example pio/7_segment

[mjcross]

1. Adds a new example showing how to control a multiplexed 7-segment LED display with the PIO.
2. Includes my previous fix for pio/onewire/onewire_library/CMakeLists.txt, as I inadvertently deleted my previous fork (apologies if this is unnecessary and/or duplicative)

[lurch]

Includes my previous fix for pio/onewire/onewire_library/CMakeLists.txt, as I inadvertently deleted my previous fork (apologies if this is unnecessary and/or duplicative)

IMHO it'd be better for these to be separate PRs, as they "fix" entirely different things.

[lurch]

Adds a new example showing how to control a multiplexed 7-segment LED display with the PIO.

I've just had a quick read through the code; IMHO this looks quite elegant, and is certainly a good demo of why using PIO is better than the bit-bashing that'd be needed on other MCUs 😎