BREAKING CHANGE: With the earlier drop of Merge as a separate class,
createMergePair() became just a convenience wrapper with confusing
behaviour. For the sake of simplicity, it's easier to drop it completely
and replace calling code with:

```diff
-doc.anchors.createMergePair(node)
+doc.createPair('<<', doc.anchors.createAlias(node))
```

BREAKING CHANGE: All of the node-creating methods are now collected on
the Document, making the API easier to use and understand.

```diff
-doc.anchors.createAlias(node)
+doc.createAlias(node)
```

BREAKING CHANGE:
- The `source` of an alias is changed from a direct reference to
  the aliased node to its `anchor` string representation instead.
- The `anchors` member of the Document class is dropped.
- Anchors are now stored in the `anchor` property of Scalar and
  Collection nodes.
- Assigning the same anchor to multiple nodes will no longer
  automatically rename an earlier anchor with the same name.

BREAKING CHANGE: Previously, `replacer` was passed as an option to
createNode(). Now it's an optional second argument, matching the API
used by the Document constructor.

BREAKING CHANGE: The behaviour of doc.createAlias() changes a bit; now
it checks that any anchor name that it assigns to the target node is
unique in the document.