Function sharpen::tree_rollup::rollup_tree

pub fn rollup_tree<T, I>(
    input: I, 
    mapping: &BTreeMap<usize, usize>
) -> Option<impl Iterator<Item = T>> where
    T: Node,
    I: IntoIterator<Item = T>, 

Roll up a tree given as a flat structure and a mapping {from child to parent} into a hierarchical structure.

Invariants for the arguments:

• The largest index in mapping should be inside of the bounds of input.
• This means that any parent must come before it's children inside input.

Return value:

• None: Detected duplicated usage of id's (probably the mapping was invalid).
• Some(collected): The rolled-up tree, with only top-level children left at top-level.