Add support for selecting pin alternate functions from python.

Converts generted pins to use qstrs instead of string pointers.

This patch also adds the following functions:
pyb.Pin.names()
pyb.Pin.af_list()

dir(pyb.Pin.board) and dir(pyb.Pin.cpu) also produce useful results.

pyb.Pin now takes kw args.

pyb.Pin.__str__ now prints more useful information about the pin
configuration.

I found the following functions in my boot.py to be useful:
```python
def pins():
    for pin_name in dir(pyb.Pin.board):
        pin = pyb.Pin(pin_name)
        print('{:10s} {:s}'.format(pin_name, str(pin)))

def af():
    for pin_name in dir(pyb.Pin.board):
        pin = pyb.Pin(pin_name)
        print('{:10s} {:s}'.format(pin_name, str(pin.af_list())))
```
Regarding the af='xxx' argument.  Could we make it similar to the mode= argument, in that the values are constants/integers.  You would have a bunch of pin AF constants defined, eg, as:
```
pyb.Pin.TIM2_CH3
pyb.Pin.USART2_TX
```
Then you do `af=pyb.Pin.TIM2_CH3`.  Is this possible?  The ST HAL definitions are named `AF1_TIM2` and are just constants, and the constant does not depend on which pin it's associated with.  So `AF1_TIM2` is valid for many pins, but means something different for each given pin (eg means TIM2_CH2 for PA1, TIM2_CH3 for PA2, etc).  And an af list would be, eg:
```
[<PinAF 1 AF1_TIM2>, <PinAF 2 AF2_TIM5>, <PinAF 3 AF3_TIM9>, <PinAF 7 AF7_USART2>]
```
Along with simplifying the implementation, this would reduce by quite a lot the number of interned qstrs.