Ord wrappers for Hex

[ManevilleF]

Proposal to implement ordering for Hex coordinates (See #146)

Add wrappers around Hex implementing PartialOrd and Ord

• ordering by length: OrdByLength
• ordering by x then y: OrdByXY
• ordering by y then x: OrdByYX

EDIT: I added an explicative section in the README

[alice-i-cecile]

Hmm. I think this is mildly useful, but not worth including in the library. These are really trivial to add to end user code.

I would consider adding the OrderByLength impl to Hex itself (since it's a good default + in many cases you don't care what the ordering is, just that it is orderable).

[ManevilleF]

I'm not sure if there is a good default, like if you retrieve a hex ring around zero all coordinates will have the same length while not being equal, only YX or XY would give consistent results with Equality comparison