Think of interface for lazy propagation

It's possible to delay operations in `Treap` (like: _add `x` to every element) to have `O(log n)` time. We just need to think how to do this...

Review the Treap implementation and its current operation interface; the issue names no files or tests. The work is complete when a concrete lazy-propagation interface for operations such as adding x to every element is defined and its O(log n) behavior is verified.

Written by the indexing model from the issue text.