Add a GetIterator method to treeMap etc

[InsZVA]

I think sometimes we use treemap to search continusly data, e.g.:
"search 20 items large than 50",
I think some code like below can solve it:

// Get and return iterator
func (m *Map) GetIterator(key interface{}) (iter Iterator, found bool) {
	_iter, err := m.tree.GetIterator(key)
	return Iterator{_iter}, err
}

// Get and return iterator
func (tree *Tree) GetIterator(key interface{}) (iter Iterator, found bool) {
	node := tree.lookup(key)
	if node != nil {
		return Iterator{
			tree,
			node,
			between,
		}, true
	}
	return
}

[InsZVA]

And a function GetOrNext/GetOrPrev which try to get the key or the next key in finding may be useful for union-indexing in database design.

[StarpTech]

Similiar API like https://github.com/google/btree would be great.

[emirpasic]

"search 20 items large than 50",

There are two ways now of answering such questions with GoDS (efficient and inefficient way) after #189 using enumerable (creates a new list) or using the efficient iterator NextTo() function. Working example:

package main

import (
	"fmt"
	"github.com/emirpasic/gods/lists/arraylist"
	"math/rand"
)

// "search 20 items larger than 50"
func main() {
	// Generate some random data
	list := arraylist.New()
	for i := 0; i < 1000; i++ {
		list.Add(rand.Intn(100))
	}

	// Inefficient way of selecting all elements (enumerable select function)
	newList := list.Select(func(index int, value interface{}) bool {
		return value.(int) > 50
	}).Values()[:20]
	fmt.Println(len(newList), newList)

	// Efficient iterator way
	it := list.Iterator()
	seek := func(index int, value interface{}) bool {
		return value.(int) > 50
	}
	for count := 0; count < 20 && it.NextTo(seek); count = count + 1 {
		fmt.Println(it.Value())
	}
}

Let me know if the above solves the needed cases, otherwise I'll reopen the issue