Binary Search Tree matrix and vector classes

This implements the sparse matrix and vector classes mentioned in Ewin Tang's paper A quantum-inspired classical algorithm for recommendation systems. I learned of that paper from a blog post by Scott Aaronson.

The data structures are not original to Tang; apparently they were used earlier by Kerenidis and Prakash.

The vector class supports storing w sparse entries in an n-dimensional space in O(w log^2 n) space, reading/writing in O(log^2 n) time, computing the norm of the vector in O(1) time, and sampling the coordinates of a vector, weighted by the square norm of the entries, in O(log^2 n) time.

The matrix class supports similar operations; see Tang's paper for details.