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()
pyb.Pin.gpio()

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())))
```

1 files changed, 1 insertions, 0 deletions

diff --git a/stmhal/mphal.h b/stmhal/mphal.h
index 4e9a8b2bb..33407622c 100644
--- a/stmhal/mphal.h
+++ b/stmhal/mphal.h
@@ -5,3 +5,4 @@
 #define GPIO_read_pin(gpio, pin)        (((gpio)->IDR >> (pin)) & 1)
 #define GPIO_set_pin(gpio, pin_mask)    (((gpio)->BSRRL) = (pin_mask))
 #define GPIO_clear_pin(gpio, pin_mask)  (((gpio)->BSRRH) = (pin_mask))
+#define GPIO_read_output_pin(gpio, pin) (((gpio)->ODR >> (pin)) & 1)