WIP => HACK ALL THE THINGS

Starting off with a TDD approach with history.c after I realized that
readline.c and readline.h are full of crazy. Given that I'd like to
implement the minimum viable parts of readline initially and flesh it
out later, it seems like identifying the useful parts of libreadline and
marking everything else as not implemented is probably the easiest way of
achieving something useful in a short amount of time.
Re-implement history as a ring buffer

Seems like a pretty good idea. The interface is a little clumsy,
but I'm willing to live with it for now.