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Like Maybe, but using unboxed sums.

Use with care. Using a unboxed maybe is not always a win in execution *time* even when allocations go down. So make sure to benchmark for execution time as well. If the difference in *runtime* for the compiler is too small to measure it's likely better to use a regular Maybe instead.

This is since it causes more function arguments to be passed, and potentially more variables to be captured by closures increasing closure size.