Add/finish Ord instance for Records

[LiamGoodacre]

Fixes #176

It's very annoying that we didn't make this instance, especially when it disrupts upgrading in some cases: removal-of-deriving-for-records-in-generics-rep#3.

[garyb]

I'm for it 👍

Anyone feel strongly against it for any reason? Speak now or forever hold your peas.

[hdgarrood]

I’m slightly uneasy about the potential for people to use a type synonym for a particular record type and expect its Ord instance to use the order written in the type synonym declaration, as was discussed somewhere a while ago (I can’t be bothered to hunt for it right now). This is a fairly minor objection though, and instance is clearly useful so 👍

[natefaubion]

@hdgarrood I don't think that's any different than people using Ord deriving and expecting the compiler to use the order of the fields in the data declaration. Edit: To clarify, for records.