Delimited continuations

[rrthomas]

Adding static continuations with shift/reset to Ark allows me to implement exceptions, generators, coroutines etc.

For JavaScript-based implementations, there's a limitation: continuations are one-shot. The immutagen library hacks around this: https://github.com/pelotom/immutagen with a quadratic performance penalty.

[mk270]

I'm going to need to be able to use chains of lazy generators, as I can in Python