Incremental indexing

[irevoire]

Fix #21

• add_item keeps track of the items that have been modified in a roaring bitmap in the metadata
• n_item as a roaring bitmap in the metadata
• About preprocess:
  • We can probably compute the max-norm on the fly while adding the items
  • If the max-norm doesn't change (from a doc addition OR deletion), then we don't need to run the second part of the pre-processing on all the nodes but only on the new nodes
• To insert the new documents, we split the IDs we're supposed to insert on every split node between the left and right
• If we deleted too many items in a descendants node, we should try to look at the split node above us to merge both branch into a single descendants node
• Once we're able to insert the items in a descendant/item leaf, we need to check its size and create a new split node if needed
• Do we need to re-compute the normal of all the impacted nodes?
• If the number of items increases, we may need to create new trees
• Fix append_item by moving the item Node mode at the end of the enum