Move Expr out of Boolean.Parser

[ARandomOWL]

By the way, I think it makes sense to move the definition of Expr to Tuura.Boolean. It's strange to see it defined in Parser. Note that this may break Plato, so it will need to be fixed too.

If we move Expr to Tuura.Boolean, we get a cyclic dependency between Boolean and Boolean.Parser. Does it make sense to move Expr and its instances into Boolean.Abstract or similar, as in Plato? Also, Boolean is starting to get very cluttered with data types and instances.