Detect number of return values in specs

I'm not sure whether this is a general specl feature, or stdlib-specific, but I feel the problem fixed by a14c5df of a function returning too many values should have been detected automatically.

Agreed, and I have a Specl rewrite (a stack of 30 uncommitted patches) that handles this a a ton of other little niggles I've been annoyed by over the last several months :-)

For now, I'm pushing several necessary changes down into stdlib first.