Move mutation processing into it's own class (#223)

* Move mutation processing into it's own object.

This should stand on it's own as a refactor, but is intended as a basis
for exposing the new MutationBuffer object to further outside control e.g.
to 'mute' or batch up mutation emission when the page becomes inactive
from a https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API
point of view

* The `processMutations` function needed to be bound to the `mutationBuffer` object, as otherwise `this` referred to the `MutationObserver` object itself

* Neglected to add this output of `npm run typings`

* Get around the binding problem by using Arrow function expressions

* Prettier formatting

src/types.ts

@@ -166,6 +166,16 @@ export type hooksParam = {
   styleSheetRule?: styleSheetRuleCallback;
 };
 
+// https://dom.spec.whatwg.org/#interface-mutationrecord
+export type mutationRecord = {
+  type: string,
+  target: Node,
+  oldValue: string | null,
+  addedNodes: NodeList,
+  removedNodes: NodeList,
+  attributeName: string | null,
+}
+
 export type textCursor = {
   node: Node;
   value: string | null;