This commit is the result of many days of picking vm apart and putting
it back together again. The result is an implementation which separates
compile and runtime into separate steps, and which functions (more)
correctly in the face of recursion.
Pretty much all aspects of vm have been modified or deleted, so it's not
even really worth it to describe specific changes. Just pretend this is
the original implementaiton and the old one was never done.
Getting `recur` to work required adding an Operation argument to a bunch
of places in a really hacky way. I don't like it at all, but I'm also
kind of out of mental energy to figure it out properly. The fibonacci
demo in the README _works_, at least, though I don't think it's actually
tail recursive.
The base graph implementation has been moved into its own package,
`graph`, and been made fully generic, ie the value on each vertex/edge
is a parameterized type. This will allow us to use the graph for both
syntax parsing (gg) and runtime evaluation (vm), with each use-case
being able to use slightly different Value types.
