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())))
```
I think using AF2_TIM5 doesn't need special handling (ie a special range): AF2_TIM5 is equivalent to the integer 2.  Just like OUT_PP etc are convenient constants, so would be AF2_TIM5 etc.