Improved PIO aided triggering (with a delay)

[nilswiersma]

Summary of changes:

• Add a configurable delay to the PIO trigger program (i.e. fast_trigger). This (and the pulse duration) are counted in 125MHz cycles.
• Use side for controlling outputs instead of set instruction.

One downside with fast_trigger is that there is currently not any kind of timeout implemented, so if the input trigger never comes, the serial interface hangs there forever.

Another maybe downside in general is that serial interface is becoming a bit convoluted with all the options and inputs. For example, now there are two configurable pulse_times, one that goes into a sleep_us and one that goes into the PIO program.

[nilswiersma]

Is there anything I could do to help you merge it?

[colinoflynn]

Oops, remembering the existence helps! Merging and will undo if it was a terrible idea, seems sane. Thanks for PR & ping again!