Boost performance of List#car, #cdr, #cadr, #cdar, etc etc

In a little micro-benchmark I tried, this gave a 9x performance boost to `#caddr`. `#car`, `#cdr`, `#cadr`, `#cdar` (and so on) should all show similar speed-ups.

I'm opening a PR so others can comment on code style before the changes are merged in.
Why code evaluation instead of using `define_method?`