COLLECTIONS-873: create PatriciaTrieSet

[ekuvardin]

According to https://issues.apache.org/jira/browse/COLLECTIONS-873

MR is basically a draft or discussion of what to do.

I make PatriciaTrieSet based on PatriciaTrie under the hood.

Below I type what I don't like. Please give some advice.

The main problem is when I access PatriciaTrie, I have to return SortedSet.
But all collections under the hood of PatriciaTrie don't use SortedSet.
Thus, I wrap call under TreeSet.

For example

@Override
    public SortedSet<String> headSet(String toElement) {
        return new TreeSet<>(trie.headMap(toElement).keySet());
    }

This is overwhelming because the keySet is already sorted, and adding to a TreeSet also takes time.

To improve the situation, I make two approaches:

1. Fully rewrite PatriciaTrieSet, copying logic with bit operations to the corresponding class.
   The drawback is that a change (bug) in bit operations leads to a rewrite in PatriciaTrieSet and PatriciaTrie independently.
2. Extend the internal PatricaiTrie class to implement NavigableMap (PrefixRangeMap and RangeEntryMap + some other classes).
   public interface NavigableMap<K,V> extends SortedMap<K,V>
   From NavigableMap, I can get navigableKeySet and work with it in PatriciaTrieSet.

[garydgregory]

Hello @ekuvardin
The build fails, it seems you did not read or follow the steps in https://github.com/apache/commons-collections/blob/master/.github/pull_request_template.md