The tree creator function. Optionally, the base size can be provided at compile time, however, the best results are achieved with the default base size of CHAR_BIT * size_t.sizeof
define the absence of a key to be -1.
A van Emde Boas node implementation
Thomas H. Cormen, Clifford Stein, Ronald L. Rivest, and Charles E. Leiserson. 2001. <em>Introduction to Algorithms</em> (2nd ed.). McGraw-Hill Higher Education. the idea of using bit operations was reused from the C++ implementation found at http://www.keithschwarz.com/interesting/code/van-emde-boas-tree/