Major interface overhaul - 2.0.0

• Major interface-changes as proposed by Erik Domke.
• Removal of async tree
• Removal of public Rebuild-functionality (will be handled internally)

public interface IRangeTree<TKey, TValue> 
    : IEnumerable<RangeValuePair<TKey, TValue>>
{
    IEnumerable<TValue> Values { get; }
    int Count { get; }

    IEnumerable<TValue> Query(TKey value);
    IEnumerable<TValue> Query(TKey from, TKey to);

    void Add(TKey from, TKey to, TValue value);
    void Remove(TValue item);
    void Remove(IEnumerable<TValue> items);
    void Clear();
}

var tree = new RangeTree<int, string>()
{
    { 0, 10, "1" },
    { 20, 30, "2" },
    { 15, 17, "3" },
    { 25, 35, "4" },
};

// Add method:
tree.Add(0, 10, "1");

var results1 = tree.Query(5);     // 1 item: [0 - 10]
var results2 = tree.Query(10);    // 1 item: [0 - 10]
var results3 = tree.Query(29);    // 2 items: [20 - 30], [25 - 35]
var results4 = tree.Query(5, 15); // 2 items: [0 - 10], [15 - 17]