Function child_node_id 

pub fn child_node_id(
    doc: &EureDocument,
    parent_id: NodeId,
    segment: &PathSegment,
) -> Option<NodeId>

Return the child node at a single PathSegment under parent.

This is ported verbatim from the previous layout.rs so that lookups against both Map and PartialMap forms succeed regardless of which segment flavor the caller uses.