Tree Borrows: Make tree root always be `Active` and initialized

There should never be an `Active` but not initialized node in the
borrow tree. If such a node exists, and if it has a protector, then
on a foreign read, it could become disabled. This leads to some
problems when formally proving that read-reordering optimizations are
sound.

The root node is the only node for which this can happen, since all
other nodes can only become `Active` when actually used. But special-
casing the root node here is annoying to reason about, everything
becomes much nicer if we can simply say that *all* `Active` nodes
must be initialized. This requires making the root node default-
initialized.

This is also more intuitive, since the root arguably becomes ini-
tialized during the allocation, which can be considered a write.

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