Comparable and FloatingPoint types

[taylorswift]

@Jon_Hull

If we tell the compiler that it uses Double's Nan bit patterns to represent 'nil' when it is optional, then we should be able to keep full speed in most operations (with the exception of ==).

I think this would need significant work in both the standard library and the compiler builtins to make this fast, but the benefits would be so worth it. I’m tired of chasing down NaNs just because they propogate silently with IEEE floats. If it were integrated into Optionals (which i feel like it certainly could be with little performance impact, since test is pretty cheap and we would expect the NaN branch to almost never be taken) it would be natural to catch and handle the NaN right when it gets generated, instead of at the end of the algorithm.