Variant constructor experiment

[Janther]

this one started as a though experiment based on the repetitive pattern of checking an instance in order to create the correct wrapping object.

👎 We lose some typechecks as this solution is more general.

😐 The types for the constructors and the keys in the Map could use some work.
😐 YulStatement and Statement had to check manually for YulBlock and Block because it generated a cyclical dependency (I could have solved this by having the factory be called in a different file but that was a bit too much and the noise is not that big).

👍 Built size decreased by 5KB 😮

Update:
Instead of a for loop we can use a Map using the Slang class constructor as key for a Map and use that to find the corresponding AST constructor. We would lose the proper inheritance tree check that instanceof does. (slight performance increase in exchange for the instanceof match)

Also if we choose to use the class name as the key, we can use an object instead of a Map which would result in a performance improvement, but that would deviate from the current instanceof approach.
I'm open to feedback here. (slightly bigger performance increase in exchange for a much looser match)