Add FromJSONE instances for Value, Env, and related types

[byorgey]

Once we merge #2202, contexts (such as those contained in an Env) will be serialized to JSON as a single hash value. The only way to read such things back in will be to provide a mapping from hash values to pieces of a context tree, i.e. CtxMap CtxTree t for some t, which can be consulted when encountering a hash. In other words, we need instances of the form instance FromJSONE (CtxMap CtxTree t) ... as the next step towards #2107 and ultimately #50.

Some commented-out code which is an early experiment towards producing such an instance for Value can be found in Swarm.Language.JSON. Unfortunately there doesn't seem to be a way to leverage currently existing automatic code generation to help write these. We might consider actually writing some instances based on Generic instead of hand-crafting instances for each type we need.