add remove method to Trie

[phryneas]

This adds a remove (and removeArray) method to Trie.

It aims for removing a child Trie's data property, and going from there, all empty child trees from their respective parent.

If a WeakMap is encountered on any Trie, removal will be aborted, as it cannot be said safely if that WeakMap is empty or contains values.

[benjamn]

Looks good to me (with a few refactorings)!