K2 treap

An efficient k^2-treap implementation.
Features:
 * in-place construction
 * k can be specified as template parameter
 * methods: 4-sided top-K queries, 4-sided count and locate queries
Yes, it should say "smaller than 17" ;) . Thanks for spotting this.
BTW: The implementation would also work for larger k, but then the overhead of the bitvector of the succinct tree is huge.