Simplifying the `Hashable` instance for `TDCtx`.

[jonathanknowles]

The TDCtx type currently has a manually-defined Hashable instance:

swarm/src/swarm-lang/Swarm/Language/Types.hs

Lines 851 to 858 in cd7ab5c

	-- Need to write manual Hashable instance since MonoidMap
	-- does not have instances of its own.
	instance Hashable TDCtx where
	hashWithSalt s (TDCtx ctx versions) =
	s
	`hashWithSalt` ctx
	`hashWithSalt` (getSum <$> MM.toMap versions)

The release of monoidmap-hashable should make it possible to auto-derive this instance.

However, it turns out that Data.Semigroup.Sum is not (yet!) an instance of Hashable. (See haskell-unordered-containers/hashable#326.)

This means we still need to do a bit of work to auto-derive the Hashable instance for TDCtx.

Potential workarounds

I can think of two workarounds (until hashable is upgraded):

1. Define an orphan instance of Hashable for Sum. Of course, an orphan instance is not ideal. However, if the hashable package is eventually upgraded to provide such an instance, this would lead to a compile error for our orphan instance, prompting us to remove the instance, which is actually what we want.

2. Define a custom TDVersionSum type, analogous to Sum, but with a Hashable instance. Then we could write:

   data TDCtx = TDCtx
     { getTDCtx :: Ctx TDVar TydefInfo
     , getTDVersions :: MonoidMap Text (TDVersionSum Int)
     }
     deriving (Eq, Generic, Hashable, Show, ToJSON)
   
   newtype TDVersionSum a = TDVersionSum {getTDVersionSum :: a}
     deriving (Eq, Generic, Hashable, Show, ToJSON)
     deriving (Num, Semigroup, Monoid, MonoidNull) via Sum a

However, option 2 has the disadvantage that the compiler would not automatically notify us if the hashable package is eventually upgraded so that Sum becomes an instance of Hashable.

[byorgey]

Thanks for thinking through the different options here! However, the thing you could not possibly know is that I have been hard at work on the feature/import3 branch, in which TDCtx becomes a bit more complex and does not use MonoidMap any longer. So I am not very motivated to remove the custom Hashable instance since it will probably just be replaced soon anyway. However, I will leave this open for now in case it does become relevant --- won't be sure until we actually merge feature/import3.