add getLeaves method for retrieving leaf points (paginated)

[mourner]

Building on #31, this PR adds a method for a super-fast retrieval of leaf points in a cluster with pagination support. This is the second part of solving mapbox/mapbox-gl-js#3318, allowing us to create a UI where you can click on a cluster and get a paginated view of all points in it with ability to jump between pages.

Usage is as follows:

index.getLeaves(
    clusterId, // cluster_id property of the clicked cluster
    zoom,      // zoom of the clicked cluster
    limit,     // how many points to return; 10 by default
    offset);   // how many points to skip for pagination; 0 by default

Since this is basically depth-first spatial tree traversal, the call is very fast — e.g. getting 1000 points starting from the 200,000-th from a 400,000-point cluster takes just 2-3ms. Getting an array of all 400k points (limit = Infinity, offset = 0) takes 220ms, which is surprisingly fast too.

• add some tests

cc @apkoponen @bewithjonam @stepankuzmin @popovae