Implementation of "until"

[John Lato]

Replacing an infinite loop with an exception sounds like a benefit to me.
While until is technically as strict as the predicate, if the predicate
doesn't evaluate at least to WHNF it's "const True|False", so I think
David's analysis is correct.

WRT strictness analysis, I think the modified annotation means that the
final argument will be evaluated at most once. I know that further analysis
can benefit from knowledge of one-shot semantics, but without an actual
example it's hard for me to say how important that is in this case.

John L.
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