Fix cleanChild() algorithm

[rvagg]

Pulling this out of a TODO attached to cleanChild() in #52 because I think it might be the most important thing to address with the current impl:

// TODO(rvagg): I don't think the logic here is correct. A compaction should
// occur strictly when: there are no links to child nodes remaining (assuming
// we've cleaned them first and they haven't caused a cascading collapse to
// here) and the number of direct elements (actual k/v pairs) in this node is
// equal o bucketSize+1. Anything less than bucketSize+1 is invalid for a node
// other than the root node (which probably won't have cleanChild() called on
// it). e.g.
// https://github.com/rvagg/iamap/blob/fad95295b013c8b4f0faac6dd5d9be175f6e606c/iamap.js#L333
// If we perform this depth-first, then it's possible to see the collapse
// cascade upward such that we end up with some parent node with a bucket with
// only bucketSize elements. The canonical form of the HAMT requires that
// any node that could be collapsed into a parent bucket is collapsed and.

This is more than just a perf improvement and needs to be addressed - either to prove that my concerns are wrong, or get the algorithm correct so it's not pushing beyond CHAMP invariants.