PWM support

[vandys]

ESP8266 has a PWM module in machine; is there a reason it's missing on ESP32 (aside from
just time needed to get to it!). I'm looking at pulling over the ESP8266 code and adapting,
unless there's a reason it won't work?

[nickzoic]

I'm actually quite surprised by this because I thought I had already implemented it! Go for it ... probably the easiest way is to look at machine_adc.c and use that same approach, and end up with the same API as ESP8266.  Let me know if you get stuck on anything!

[vandys]

Hi, thanks. On it. The API makes it look like you can specify PWM frequency on each PIN, but
the hardware just has a common frequency (well, one high speed and one low). Do I just let
the frequency get overwritten each time? I'm thinking about just using one timer, maybe the
low speed one. Duty cycle is, of course, per PIN.

[vandys]

Oh, yes, here's the code I wrote to do a pretty fading light thing with the API.

colors.txt

[dpgeorge]

I'm actually quite surprised by this because I thought I had already implemented it!

We have DAC, but not PWM...

[mattytrentini]

I see that @vandys has submitted PR #79, just linking here for reference.

[dpgeorge]

Implemented, see #96