Interpret: Create a blackhole while evaluating a thunk

Haskell's G-machine uses a "blackhole" [1] to avoid space leaks and detect certain forms of nontermination. We can do the same quite easily.

Before evaluating a thunk, extract the value from the localtable and replace it with a "blackhole" node (containing no data). This allows the garbage collector to collect anything referenced by the thunk, which it would otherwise be unable to collect. If you have to evaluate a blackhole, just raise an error, as it indicates infinite recursion in thunk evaluation.

[1] Jones, Richard. Tail Recursion Without Space Leaks
http://reference.kfupm.edu.sa/content/t/a/tail_recursion_without_space_leaks__1003476.pdf