Implement inb().
Implement serial_initialize() and serial_write().

These work with QEMU... but they also work under the following strange
conditions:
  * Neglect to call serial_initialize().
  * Make serial_can_transmit() return true unconditionally.

QEMU is insensitive to these issues, but a physical UART would might
have trouble.  Since we have already demonstrated that QEMU is
insensitive to some potentially devastating bugs, I am not confident
that this implementation is correct.  It should be tested on real
hardware at some point!
Make cprintf print to COM1, in addition to the VGA display.

Since QEMU pipes COM1 to stdio, the host OS can now read Asbestos's
output.  This should allow for more convenient testing.
Add description and resources to serial.c.
Merge pull request #3 from jasonpr59/serial-write

Implement serial output.