Change UpdateParser to use Monad Transformers

[lispandfound]

Small enhancement to the update parser interface, use monad transformers instead a custom monad implementation. This makes it easier to understand for people reading the code base for the first time, and also means we inherit all the typeclasses mtl and transformers already give ReaderT and Maybe. Provided end users are not using the UpdateParser constructor directly (i.e. they use mkParser instead) this is a completely transparent change.

[lispandfound]

In particular, I have tested that all the examples compile without changes using the new code. The example todo bot works identically to the todo bot on master. I haven't tried the game bot.

[swamp-agr]

This is a very good observation! Thank you for contribution! I have checked instances of ReaderT in advance (they are identical).