ChildSyntaxList Struct

A node holds its children in numbered slots, and a slot may hold a whole list. This type presents both cases the same way, so a caller sees a flat sequence of nodes and tokens and never a list of its own.

Reading a child walks the slots to find it, so it costs more than an array lookup, and enumerating pays that walk once per step rather than carrying a running position from one to the next. The walk is short -- it is over the slots of the node, not over the children of a list -- and the position of a child already created is read straight off it.