Refactor HIR Value VS Expression (CoreData)

[AlSchlo]

Right now we have some unreachable states, like a CoreExpr that wraps a Map, or a Closure. Just wasted 30min on old design that can clearly be improved now that we have a better understanding of the HIR.

Solution would be to separate what a Value is and what an expression is.
Having MapValue, or MapExpression is totally fine and less confusing than dealing with a mess of generics.

EDIT: + Fix Closure span while at it like we did for LetBinding