CallListNode Modifier definition hacky

[d1m0]

Currently the CallListNode is defined as follows:

.type CallListNode = Node { id: id }
                  | Call { id: CallsiteType, target: CalleeType }
                  | Modifier { id: FunctionDefinitionId, idx: number, placeHolder: PlaceholderStatementId }
                  | BaseConstructor { mdc: ContractDefinitionId, baseIdx: number }

The Modifier branch actually serves 2 different purposes - for the case of a normal ModifierDefinition body, and also when are in the body of the actual function, we add an extra $Modifier(funId, -1, placeHolder) just to remember the exact PlaceholderStatement in the topmost modifier that we are in.

It might make sense to split Modifier into 2 branches.

(Also the printing logic for Modifier in Fact.fmt is fairly ugly)