Handling Env modifications

[jkarni]

Currently env state is not managed by the effects system.

A few things to consider:

• On the one hand it would be nice to unify the env modifications with the rest of the effects system
• On the other this means we need continuations to handle local gets and sets.
• Persistence will probably eventually just be a subscriber

[jkarni]

Arguably errors too are in a similar position, but I'm definitely punting on that problem until I see the benefits.

[jkarni]

Done, albeit in a different way than originally conceived of in this issue, in #99.