Various improvements and fixes.

• Fixed ifte and made the nonDetEffTests better.
• Fixed runC for the Yield effect.
• Added a runM handler for running an arbitrary monad (if it's the only effect left to handle).
• Added a modify function for the State effect.
• Removed the ProxyState type wasn't necessary in favor of Data.Proxy.
• Made the Writer effect work with arbitrary monoids instead of just lists.