sparse run tree level

[willow-ahrens]

The sparse run tree level would use a tree to manage runs of sublevels. We would also need to use a strategy similar to the separation level to merge the identical runs. Let's also deprecate the merging functionality of RepeatedRLELevel, and make it generic on the sublevel type.

As part of this issue, we should also make sure that common operations like A[3.14] and A[1.2:1.3] = 3 have tests