applet.interface.pulse_counter: new applet to count pulse events on pins

[TomKeddie]

I'm writing this applet for a testbench I'm building. Initial version will just edge detect on pins and increment counters.
Other future features could include

• counter reset
• per pin configurable debounce time
• pulse width measurement (min/max/median/recent)

[attie]

see #218 and #216

(depending on your specific requirements, this issue can probably be closed / continue discussion over there?... they should both be usable, but aren't cleaned up / quite finished yet)