Rewrite list with Rc rather than Box.

[hawkw]

I'm trying to write a wholly stack allocated list implementation, as described in #89, but this may not ever end up seeing the light of day, primarily due to issues relating to the list creation macro (which I think is kind of a necessary feature).

However, it should be easy to replace the boxed references in the current list implementation with reference-counted references, which should make the list persistent and lead to some performance improvements, anyway

[hawkw]

It should be possible to do something similar with State so we don't have to copy unchanged stack values. And now that we have benchmarks we can see if this actually makes it faster.