Document guarantee that hash (Arg x _) == hash x

[josephcsible]

In haskell-unordered-containers/unordered-containers#349, I'm proposing to add functions that convert between HashMaps with k as keys and ones with Arg k v as keys, without needing to rehash anything. For this to be safe, it needs to be the case that hash (Arg x _) == hash x. This is indeed the case, but the documentation doesn't currently guarantee this. It only lets us infer the weaker guarantee x1 == x2 -> hash (Arg x1 y1) == hash (Arg x2 y2), which isn't sufficient for this purpose. Can we add the stronger guarantee to the documentation?

[phadej]

Sure. That was a reason of hashable-1.3.0.0 major release, so we can as well document that behavior on the instance of Arg, for others to rely.

I don't see any reason to change the instance ever, and if there would be a reason, the change will be a breaking change, so we'd need to consider the motivation properly.

[josephcsible]

That was a reason of hashable-1.3.0.0 major release

I thought the reason for that was just to make the weaker guarantee hold, and the fact that the stronger one holds too was just a lucky coincidence.

But in any case, this is good news, thanks. I'll write a PR for this tonight.