Add mruby-esp32-pwm. - #49

Just tested this with a servo on my ESP32. Works great!

Wrt naming, I think the class should be MCPWM and the gem mruby-esp32-mcpwm. Your methods already follow the spec, and we can rework LEDC to match the spec too, so there's consistency. But I like that the class and gem names make it clear which resources on the chip are being used.

I didn't dig all the way into the MCPWM docs yet, but can each channel set its frequency independently?

For LEDC there are 4 timers shared among each group of 6 or 8 channels, so #frequency might get tricky. To abstract it away, it will need to optimize timer use, so motors all share one 50Hz timer for example, and timers no longer in use can be reused. Maybe an issue for MCPWM too?

@vickash
Thank you for your advices ! I also think this mrbgem should be renamed.

However, we believe that a class named PWM is necessary in case a program is ported from another platform (ex. mruby/c) or a beginner uses mruby-esp32.

I checked the MicroPython on ESP32 documentation and the PWM class was still present. And it uses LEDC internally.
https://docs.micropython.org/en/latest/esp32/quickref.html#pwm-pulse-width-modulation

I have two ideas.

1. Extend the methods of the PWM class described in "peripheral class common specifications" to mruby-esp32-ledc and add an alias with the class name PWM
2. Implement a new PWM class with LDEC internally

How about I create a Pull Request with idea 1, if you don't mind?

I didn't dig all the way into the MCPWM docs yet, but can each channel set its frequency independently?

Perhaps you could use more than one timer. However, the current mruby-esp32-pwm can only use one timer.

However, we believe that a class named PWM is necessary in case a program is ported from another platform (ex. mruby/c) or a beginner uses mruby-esp32.

Ah, I see. I think aliasing LEDC to PWM is a good choice then. I'll do a PR soon with those changes, and some logic to manage its timer allocation.

@vickash
Thanks. I pushed rename mrbgem commit.
Please review its code.