Originally, I'd intended for the tree to expose diverging structure to
the merger, and for the merger to fix it. That was part of why I split
`Item` into `Existing` and `Missing` variants.

However, it turns out it's simpler to resolve divergences in the tree,
and expose a well-formed structure to the merger instead. Changing
`Item` to an enum means we now need a pattern match for _every_ item,
when all we really want is a flag on the parent that says "this folder
has diverged because it has a missing child".

`Entry::divergence` already flags diverging structure for multiple
parents, so let's use it to also flag parents with missing children,
instead of forcing the merger to handle invalid structure for this one
case.

* Add missing doc comments.
* Rename reparenting methods for clarity.
* Move `Tree::{children, parent}_for_entry` to
  `Node::{children, parent}`.
* Remove optional return value from `Node::level`.
* Add `Node::is_{root, default_parent_for_orphans}`.
* Iterate over `Tree::entries` instead of
  `Tree::entry_index_by_guid`. The two should point to the same
  entries, but `entries` is more direct.
* Don't mark trees with orphans as equal.

This reverts commit b5e38d4.

* Clarify why `merge::Driver` exists.
* Make `Merger` generic over `Driver`, instead of using a trait object.
* Use `clone` instead of `to_owned`.
* Replace `Entry#is` with `std::ptr::eq`.
* Add `MergedNode::remote_guid_changed`.

In most cases, `node.2 == node.entry().divergence`, except for
orphans, default orphans, and diverging `parentid`s. This is a
surprising inconsistency that means `node_for_guid` and
`children` need to do more work to figure out if the node has
actually diverged.

This commit:

* Changes the tree to flag divergent `parentid`s at `insert` time.
* Cleans up `Tree::structure_for_insert`, to clarify what happens when
  a `parentid` is or isn't provided.
* Moves the logic for checking default folder divergences into
  `Node::diverged`.
* Replaces `EntryParents::{indices, guids}` with `EntryParents::iter()`,
  which doesn't allocate (thanks, @thomcc!).

* Clean up optionals with `.map_or(...)` and `.filter(...).map(...)`.
* Explain why we use `ptr::eq` to compare entries.

* Shorten import paths.
* Rename `Child::Existing` to `Child::Exists`.
* Rename `Divergence::Ok` to `Divergence::Consistent`, since `Ok`
  might be confused with `Result`.

We need this to apply the merged tree, when we join to the local and
remote trees.

* Fix infinite recursion in `fmt::Display::fmt()` for `Error`.
* Make `Store` generic over the error type. This allows callers to
  provide their own error types that unify with Dogear errors,
  `nsresult`s, and others, instead of requiring them to wrap their
  errors into `ErrorKind::Storage(...)`.
* Forward decoding errors from `Guid::from{utf8, uft16}()`.
* Rename `ErrorKind::GenerateGuid` to `ErrorKind::InvalidGuid`.
* Move `dogear::merge` into `Store::merge`.