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Consistently use `_children` & combine Fragment and Component diffing (-73 B) (#1658)

## Summary

* Replace `component._prevVNode` with `vnode._children`
* Combine Fragment and Component diffing

## Details

### Replace `component._prevVNode` with `vnode._children`

Previously, `_children` was a cache for a normalized `props.children`. With a DOM VNode and Fragment VNode, you could access the children of the VNodes on the this `_children` property, but on VNodes for components, you need to go through `vnode._component._preVNode`.

This PR formalizes the concept behind `_children`. It is now the container for the rendered children of all VNodes. If you think of `props.children` as input to render functions, `_children` is the output of the `render`. And DOM VNodes and Fragments are just render functions that return `props.children`. The implementation is a little different (DOM VNodes don't have a render function, per se), but this concept applies.

### Combine Fragment and Component diffing

Previously, if `diff` was called with a Component, it would call `c.render`, coerce the result to a single VNode (e.g. convert arrays to a Fragment), and then call `diff`.  However, building on the `_children` change (and the concept mentioned there), we now instead treat Components as always returning arrays, and coerce result of render into a children of VNodes to continue rendering. So, `diff` always calls `diffChildren` after diff'ing a VNode, regardless of its type. Another way to think about this is that everything is Fragment now :smile:

Being able to depend on `diffChildren` -> `diff` -> `diffChildren` for all VNodes allows us to do operations that require knowing the `parentVNode` in `diffChildren` without having to special case the `diff` -> `diff` situation. For example, there is now only one place where oldVNode matching happens: `diffChildren` (we can remove some code from the top of `diff`). Further, mounting and unmounting DOM onl... (continued)

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