ephemeral lazy lists

[matthewleon]

Sometimes we create lists that will only be traversed a single time. Such a list could implement the Lazy class without actually using the purescript-lazy Lazy datatype. It would probably be faster, and would probably avoid FFI. I'm curious to try and benchmark.

It would be particularly interesting to try to see if it's possible to construct this type in a way that lets GC kick in progressively. Not sure if that would be possible without some FFI hacks, though.