#### Is this adding or improving a _feature_ or fixing a _bug_?
Feature.
#### What's the new behavior?
This pull request adds paths to `Range` objects, including the selection. The paths and keys are kept in sync automatically, so that you can use whichever is ideal for your use case.
This should allow us to use paths for lots of the internal logic, which are much quicker to work with than keys since they avoid having to lookup the key in the document and can just traverse right to the node in question.
#### How does this change work?
`Range` objects have two new properties:
```js
range.anchorPath
range.focusPath
```
(Eventually these will be `range.anchor.path` and `range.focus.path` when points are introduced.)
When operations occur and whenever ranges are created/normalized, the paths are updated and kept in sync with the keys.
#### Have you checked that...?
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* [x] The new code matches the existing patterns and styles.
* [x] The tests pass with `yarn test`.
* [x] The linter passes with `yarn lint`. (Fix errors with `yarn prettier`.)
* [x] The relevant examples still work. (Run examples with `yarn watch`.)
#### Does this fix any issues or need any specific reviewers?
Fixes: https://github.com/ianstormtaylor/slate/issues/1408
Fixes: https://github.com/ianstormtaylor/slate/issues/1567
