Some BSD socket functions don't return error numbers in errno namespace, but
rather in other error namespaces. CPython resolves this by using OSError
subclasses for them. We don't do that so far, so there's ambiguity here.

Well, Python3 also defines an attribute for that, but that's bloat.

Exact behavior of typecodes may be not yet enforced.

Instead of pyb.switch() as a function, it's more consistent (with
respect to all the other modules and peripherals) to have
pyb.Switch() create a switch object.  This then generalises to having
multiple switches.  Call the object to get its state.  Use sw.callback
to set the callback function for when the switch is pressed.

It should be a noun, since it supports getting and setting depending on
the number of arguments given.

Simple but functional timer control.  More sophistication will
eventually be added, or for now just use direct register access :)

Also added pyb.freq() function to get MCU clock frequencies.

It's really a UART because there is no external clock line (and hence no
synchronous ability, at least in the implementation of this module).
USART should be reserved for a module that has "S"ynchronous capabilities.

Also, UART is shorter and easier to type :)

This is a one-liner fix.  It gets the class-super.py test passing, but
is probably not a complete fix.

modffi: Fix how we call `pkg-config`

The three classes I2C, SPI and USART now have a fairly uniform (Python)
API.  All are constructed, initialised and deinitialised in the same
way.  They can have most of their parameters set, using keyword arguments.
All have send and recv (although slightly different with I2C requiring an
address in master mode).  recv can do inplace receiving (ie store the
data in a previously-created bytearray).

It's just polling mode at the moment, but interrupt and DMA would be
nice to add.

TODO: Never "optimize" includes any more!