Expose methods for mutable trees

[milibopp]

Currently, methods such as insert are private, since (1) they cannot guarantee the internal invariants of a tree completely, and (2) their signature is inconsistent between Tree and PureTree.

However, it may well be sensible to access the tree mutably after its construction. Therefore, it is necessary to provide a public interface that fulfills the missing properties mentioned above.

Furthermore, there may be more methods than insert to consider, e.g.

• remove(filter) removes objects based on some filter.
• collapse(filter) collapses nodes to empty nodes based on some filter.
• extend(iterator) inserts all elements from the iterator into the tree. (stdlib trait Extend)
• …

[milibopp]

In order to follow the immutable-by-default principle, all these methods should come in two flavours:

• A method that mutates the tree:

fn collapse_mut<F: FnMut(&Node) -> bool>(&mut self, filter: F);

• A method that makes a copy of the tree with the appropriate changes

fn collapse<F: FnMut(&Node) -> bool>(&self, filter: F) -> Self;