Use float for ordering, add Ordering module

[glennsl]

Naively converting JS code to rescript may result in unexpected and hard to track down bugs because the ints used for ordering overflows much easier than JS numbers.

This does two things:

1. Uses floats instead of ints for ordering to avoid premature overflows.
2. Adds an Ordering module to make comparison function code more readable, and more convenient to convert from ints.

[zth]

This looks great to me! @cknitt ?

[cknitt]

I already suggested using floats once, see #74, but @cristianoc was not in favor of that at the time.

[zth]

Right, I forgot about that conversation it seems... 🙈

[glennsl]

I already suggested using floats once, see #74, but @cristianoc was not in favor of that at the time.

Huh, I'd forgotten about that too. Seems I've changed my mind. Maybe @cristianoc has too!

[cristianoc]

I already suggested using floats once, see #74, but @cristianoc was not in favor of that at the time.

Huh, I'd forgotten about that too. Seems I've changed my mind. Maybe @cristianoc has too!

I just made an observation not a recommendation.
Happy to go with whatever decision now that the relevant facts are known.

[jmagaram]

Please see #150