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Do typeof string check before looking for String constructor (#4198)

In `constructNewChildrenArray` we look for the `String` constructor to support #4151 (i.e. strings created with `new String`). However, doing this check first leads to a megamorphic deopt in `constructorNewChildrenArray` since V8 has many different internal types for strings, leading to a megamorphic access for `childVNode.constructor` in the common case (normal strings and VNodes).

This PR adds back the `typeof childVNode == 'string'` check to catch normal strings first before falling back to `childVNode.constructor == String` to check for manually constructed strings.

Commits:
* Add back typeof string check in diffChildren (+5 B)
* Store matching oldVNode in variable in constructNewChildrenArray  (-4 B)
* Inline insertion checks into skew block (-3 B)

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