Growable fiber stacks

[dhil]

Currently, the size of a fiber stack is fixed, meaning a fiber-based computation may run out of stack space. On one hand the fixed size model is nice because it is simple and does not occur an overhead on other constructs. On the other hand it can be quite wasteful as no size-fits-all, so conservatively we have to allocate enough space for the largest fiber-based computations, which obviously means we may waste considerable amounts of memory for small fiber-based computations.

We should experiment with different stack growing techniques: explicit stack size check in function preludes, virtual memory gstacks, etc.