Implement a ring buffer.

This will be used in an upcoming commit for the keyboard driver.

Hopefully, it will be more broadly appicable, too.
Implement keyboard driver.
Implement read_string().
Implement input_string().

This is the input equivalent of cprintf.

This commit involved updatign cprintf.h to expose the kernel
print-character routine.
Implement the kernel echo prompt!

This prompt demonstrates that the bare-minimum IO system works.
Add forgotten import.
Merge pull request #4 from jasonpr59/keyboard

Implement keyboard driver.