Parse S-expressions into the abstract syntax tree

Parse S-expressions into the abstract syntax tree

While doing this, we handle name resolution and "upvar" captures. The
name resolution context "Env" forms a stack that has a reference to the
outer environment at each level. A new level is pushed at each lambda
that's introduced. The environment includes a list of "upvars" that have
to be copied into the closure, which is updated every time a variable
lookup finds the variable in an outer environment instead of the local
environment. This process has to use a RefCell because you can't make
the lifetimes work out with a chain of mutable references.

The parsing aside from that environment stack is relatively
straightforward parsing.