This is an attempt at a very simple Lisp (or maybe Scheme, not sure yet). Currently the only working things are '+', 'def' and ints. Examples:
>>> (def foo (+ 1 2 3 4))
result: Ok(Int(10))
>>> (+ foo 1)
result: Ok(Int(11))
...so it's a barely functional calculator ATM. I learned a whole lot about Rust, though, especially about mixing boxes with values/guards/cells/etc. Thanks to #rust for that.