Change:
In particular, MLS maintains the members' views of the tree in such a way as to maintain the _tree invariant_: <del> In other words, each member holds the private keys for nodes in its direct path, and no others. </del> <ins> In other words, if node is not blank, then it holds a key pair, and the private key of that key pair is known only to members holding leaves below that node. The reverse implication is not true: A leaf below an intermediate node might not hold the private key for the node.  Such a leaf is called an _unmerged_ leaf, since encrypting to the subtree below the node requires encrypting to the node's public key as well as the unmerged leaves below it.  A leaf is unmerged when it is first added, because the process of adding the leaf does not give it access to all of the nodes above it in the tree.  Leaves are "merged" as they receive the private keys for nodes, as described in ratchet-tree- updates. </ins> 5.4.

Discussion:
This PR attempts to implement the "non-destructive add" proposal discussed at IETF 105. Instead of blanking nodes on add, we keep track of "unmerged leaves" below each intermediate node. This involves a bit more complexity in the tree state that is passed around, but avoids unnecessary blank nodes.
As discussed in the interim: Remove processing should be updated such that for each node in the removed node's direct path: If the removed leaf is in for the node, then just remove it from that list Otherwise, blank the node
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