Contexts that can be serialized + deserialized while retaining and ex…

…plicitly representing sharing (#2202)

Towards #2107.  Instead of simply storing a `Map`, contexts now also store a structured tree representing how it was built, and cache homomorphic hash values so contexts can be compared + stored by hash.  This allows us to serialize contexts into a compact form that never duplicates any context, then deserialize them in a way that restores all the sharing.

The big thing that is left is to make `FromJSONE` instances for `Value` and `Env` (and anything which contains them).  I was working on these but got bogged down, so decided to not let the perfect be the enemy of the good.  You can see some commented-out code towards a `FromJSONE Value` instance.  But we don't use any such instances yet, so I think this can be merged as is, and the rest worked out later.

src/swarm-engine/Swarm/Game/CESK.hs

@@ -169,9 +169,6 @@ data Frame
 instance ToJSON Frame where
   toJSON = genericToJSON optionsMinimize
 
-instance FromJSON Frame where
-  parseJSON = genericParseJSON optionsMinimize
-
 -- | A continuation is just a stack of frames.
 type Cont = [Frame]
 
@@ -184,7 +181,7 @@ type Addr = Int
 -- | 'Store' represents a store, /i.e./ memory, indexing integer
 --   locations to 'Value's.
 data Store = Store {next :: Addr, mu :: IntMap Value}
-  deriving (Show, Eq, Generic, FromJSON, ToJSON)
+  deriving (Show, Eq, Generic, ToJSON)
 
 emptyStore :: Store
 emptyStore = Store 0 IM.empty
@@ -269,9 +266,6 @@ data CESK
 instance ToJSON CESK where
   toJSON = genericToJSON optionsMinimize
 
-instance FromJSON CESK where
-  parseJSON = genericParseJSON optionsMinimize
-
 -- | Is the CESK machine in a final (finished) state?  If so, extract
 --   the final value and store.
 finalValue :: CESK -> Maybe Value

src/swarm-engine/Swarm/Game/State/Substate.hs

@@ -132,7 +132,7 @@ data REPLStatus
     --   entered.  The @Maybe Value@ starts out as 'Nothing' and gets
     --   filled in with a result once the command completes.
     REPLWorking Polytype (Maybe Value)
-  deriving (Eq, Show, Generic, FromJSON, ToJSON)
+  deriving (Eq, Show, Generic, ToJSON)
 
 data WinStatus
   = -- | There are one or more objectives remaining that the player