Systas eval.c

First, holy crap...  Tom, do you actually understand what's going on
in there?!

Okay, now that I've got that out, looking at the code I can see where
a lot of the ideas in some of Pika's design notes come from (leaving
DO as a semi-built-in), but with all the hairy bit manipulation stuff
(ugh, tags.h gives me a head ache) how much of eval.c can we actually
plan on using?  (FFI style issues aside.)

Also, I know Tom's a big fan of SCM-style evaluation (rewriting sexps
through evaluation) because of its low latency properties, but Pika
seems interested in supporting a byte-code evaluating virtual machine.
It seems like these are conflicting implementation ideas aside from
something like having both virtual-machine-pika and
tasty-low-latency-pika executables, but maybe I'm missing some complex
control shifting mechanism?

-jivera