Provides the iradix package that implements an immutable radix tree.  The
package only provides a single Tree implementation, optimized for sparse nodes.

As a radix tree, it provides the following:

 * O(k) operations. In many cases, this can be faster than a hash table since
   the hash function is an O(k) operation, and hash tables have very poor cache
   locality.
 * Minimum / Maximum value lookups
 * Ordered iteration

A tree supports using a transaction to batch multiple updates (insert, delete)
in a more efficient manner than performing each operation one at a time.

For a mutable variant, see go-radix.