0m0.413s: Make $code be a <program>.

This new limited type lets us remove a type check in a key inner loop.

real  0m0.413s
user  0m0.409s
sys 0m0.003s
Minor cleanup.
0m0.398s: Better representation for *env*.

Using a limited vector of vectors here lets us remove some of the
ugly hacks from lref / lset.

real  0m0.398s
user  0m0.393s
sys 0m0.004s