Rename Range to Leaf, and Selection to Range (#1231)

* rename Range to Leaf

* rename Selection to Range

* add findDOMRange, findNode, findRange helpers

* refactor to remove findDropPoint util

* revert findDOMNode to throwing errors

* export new helpers, fix linter

* update docs

* update examples

docs/concepts/the-document-model.md

@@ -31,4 +31,4 @@ Note that even though the node is "void", it will still have an empty text node
 
 One constraint of Slate documents is that the leaf nodes are always `Text` nodes. No `Block` or `Inline` node will ever have no children. It will always have at least an empty text node. (However, you can _render_ text-less nodes, see the [Void Nodes](#void-nodes) section above!)
 
-This constraint means that [`Selections`](../reference/slate/selection.md) can always refer to text nodes, and many text-node-level operations are always "safe" regardless of the tree's structure.
+This constraint means that [`Ranges`](../reference/slate/range.md) can always refer to text nodes, and many text-node-level operations are always "safe" regardless of the tree's structure.

docs/concepts/the-selection-model.md

@@ -1,7 +1,7 @@
 
 # The Selection Model
 
-Slate keeps track of the user's selection in the editor in an immutable data store called a [`Selection`](../reference/slate/selection.md). By doing this, it lets Slate manipulate the selection with changes, but still update it in the DOM on `render`.
+Slate keeps track of the user's selection in the editor in an immutable data store called a [`Range`](../reference/slate/range.md). By doing this, it lets Slate manipulate the selection with changes, but still update it in the DOM on `render`.
 
 
 ### Always References Text