BP Bucket-mMT Support

[Cydhra]

Add support for partitioned Min-Max-Trees as described in Section 3 of Cordova & Navarro. The partitions will reduce the size of the mMTs, especially because it allows the trees to use much smaller pointer sizes. Small Trees only require one mMT, so this approach has no downside for small to medium-sized trees, and introduces a small overhead for queries over large distances in large trees.

If it turns out that this approach has no benefits in practise, we revert to the naive mMT implementation.

This will change the struct layout of the BP tree, and thus break the serde-compatibility. In an interest to release a working version of BP without introducing breaking changes later, we will resolve #15 without serde support.

[Cydhra]

serde support has been added in 1.6.2 because this issue is projected to take longer to implement. It will be added within a major release instead and will break serde compatibility.