3986 - bring back delimited continuations

They're back after a long hiatus: commit 2295 in Oct 2015.

I'm not convinced anymore that this is actually a correct implementation
of continuations. Issues on at least two fronts:

a) These aren't safe yet. Since continuations can be called multiple
times, we need to disable reclamation of locals inside a continuation.
There may be other type- or memory-safety issues. However, delimited
continuations at least seem possible to make safe. Undelimited
continuations (call/cc) though are permanently out.

b) They may not actually be as powerful as delimited continuations.
Let's see if I can build 'yield' out of these primitives.

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