Ord instances for integer wrappers e.g. U64

[jjl]

It would be nice if they could be compared directly rather than having to .get() them before comparison. That would allow deriving Ord over structs using them that are in disk format.

[joshlf]

If memory serves, I tried this when I initially introduced those wrappers, and it caused a lot of type inference issues. I'll take a stab at it again when I get a chance to confirm that that was the reason.

[joshlf]

Closed in #915.