Remove effect monad from expressions #9

Right now the expressions contain lambdas directly as Haskell functions which in previous iterations forced the effect handler to live inside of Expr. Via a little bit of clever unsafe IO on effects that are only reads or throws, we were able to circumvent this. Still, it is really not ideal! For one, it makes it hard to properly support pruning as this effect would be broken to add into this unsafe IO wrapper. Hence, this blocks issue #7. Instead, we should change the Expr to not use a function internally!