Here's a mostly-working bitbang driver for SPI.  It seems to
handle SPI mode 0 just fine; I've done a bunch of flash updates
through a parport wrapper, and no bits were damaged.

Eventually I hope this will serve as a reference implementation,
showing how the various API operations map to bits on the wire.
(Contrast it to the I2C bitbangers, also.  Simpler...)

Likewise, folk looking at getting more standard SPI support on
their hardware could start with this.  It's reasonably common
for the pins used for SPI controllers to be accessible as GPIOs,
so this bitbanger might be used until a real hardware driver was
debugged and tuned.  (That's assuming there _is_ a dedicated
SPI controller to use!)

Comments appreciated.  Also testing with other SPI modes,
and potentially code tweaks to make the banging go faster.

- Dave