Instead of SparseIndices presence, split leaves on isDeleted status?

[mweidner037]

Pros:

• Remove sole dependency
• Reduce bundle size (sparse-array-rled is nontrivial)
• More exact asymptotic performance guarantees: O(log(# leaves)) for slightly more leaves, instead of O(log(# leaves) + (max size of a SparseIndices internal rep)).

Cons:

• Probably need to merge leaves for reasonable efficiency. Can be lazy (e.g. not between parents) - will always be patched up by reloading anyway.
• Might increase memory usage due to more leaves, which means duplicating LeafNode fields. Need to benchmark this.